Molecular Link Between Circadian Clocks and Cardiac Function: A network of Core Clock, Slave Clock and Effectors

Abstract

The circadian rhythm has a strong influence on both cardiac physiology and disease in humans. Preclinical studies primarily using tissue specific transgenic mouse models have contributed to our understanding of the molecular mechanism of the circadian clock in the cardiovascular system. The core clock driven by CLOCK:BMAL1 complex functions as a universal timing machinery that primarily sets the pace in all mammalian cell types. In one specific cell or tissue type, core clock may control a secondary transcriptional oscillator, conceptualized as slave clock, which confers the oscillatory expression of tissue-specific effectors. Here, we discuss a core clock-slave clock-effectors network, which links the molecular clock to cardiac function.

Graphical abstract

1. Introduction

Circadian rhythm is a cell-autonomous timing system evolved to optimize biological processes in anticipation of recurring external cues [1,2]. In the cardiovascular system physiological parameters such as heart rate, blood pressure and endothelial function show diurnal rhythmicity [3,4]. Whilst disruption of circadian rhythm is associated with cardiovascular disease (CVD) such as myocardial infarction, strokes and sudden cardiac death [5**,6,7,8**,9,10*].

Here we will discuss a core clock-slave clock-effector network which governs the cardiac function (Figure 1). The primary molecular motor for circadian rhythm is the core clock. It consists of both transcriptional activators and repressors, which rhythmically express within an autoregulatory transcription-translation feedback circuit. The core clock presides over the slave clock, a secondary oscillator that governs the gene transcription of essential downstream output effectors and enables rhythmic tissue-specific functions. The core clock may also directly regulate certain effectors. Finally, dual regulation from the core clock and slave clock is also possible.

Figure 1. The core clock-slave clock-effectors network.

This schematic diagram illustrates the proposed model of the molecular mechanism of circadian regulation in the heart. The core clock generates the primary oscillation, which is either transduced by a tissue specific slave clock or directly to a variety of downstream effectors. The slave clock allows tissue specific circadian regulation as well as integration of additional environmental inputs, such as metabolic or disease status. The effectors ultimately affe ct cardiac physiology or pathophysiology.

In this review, we will first discuss the function and regulation of the core clock and the slave clock in the heart. Kruppel-like factor 15 (KLF15) will be highlighted as an essential slave clock in heart. In the latter part, we will focus on downstream oscillatory effectors which are critical in cardiac function and have shown significant cardiac phenotypes in animal studies when their circadian regulation is disrupted. We will discuss the circadian regulation of these effectors by core clock and slave clock as well as their impact on cardiac pathophysiology.

2. The core clock

2.1. The molecular core clock

The molecular circuit of the core clock is identical across all tissues and cell types. The CLOCK:BMAL1 complex activates Cry and Per gene expression. Accumulation of cytoplasmic CRY:PER complex leads to their nuclear translocation and switches off the CLOCK:BMAL1-driven gene transcription of Cry and Per [11,12]. Moreover, CLOCK:BMAL1 complex also drives the transcription of Nr1d1/2 and Nr1f1 which encodes nuclear receptors REV-ERBα/β and RORα. REV-ERB competes with RORα at the ROR binding elements (ROREs), which suppresses and actives the transcription of Bmal1, respectively [13,14,15].

The critical function of the peripheral core clock in the heart has been directly demonstrated by the significant cardiac phenotypes in mouse models where core clock function was genetically ablated in a tissue specific manner. Mice bearing cardiomyocyte-specific Clock mutant (CCM) [16,17] and Bmal1 knockout (CBK) [18,19,20] display significant disturbance of circadian rhythm in blood pressure and heart rate accompanied with abnormal cardiac electrophysiology and function. Additionally, PER2 regulates the rhythmicity of heart rate [21] and susceptibility to ischemia-reperfusion injury (I/R injury) in mice [22,23,24,25]. Further, REV-ERBα agonists and antagonists have been shown to provide cardioprotection against heart failure in mouse models and I/R injury in both mice and humans [26,27,28,29**].

2.2. Regulation of the core clock by post-translational modification

The core clock components undergo extensive and diverse post-translational modifications, including acetylation, phosphorylation, methylation, ubiquitination, SUMOylation, O-GlcNAcylation, ADP-ribosylation and S-nitrosylation [8**,30,31,32,33,34]. Particularly, acetylation, phosphorylation, ubiquitination and O-GlcNAcylation of core clock are known to modulate circadian rhythm in heart. Acetylation of BMAL1 directly regulates the CLOCK:BMAL1 activity. In fact, CLOCK itself is an acetyltransferase and acetylates its partner BMAL1 on K536 [35], which is required for gene transactivation [31]. In contrast, deacetylation of BMAL1 by sirtuin 1 (SIRT1) suppresses the core clock-activated transcription [31,36,37]. SIRT1 also deacetylates PER2 and promotes its degradation [38]. Either loss or overexpression of SIRT1 in heart can lead to cardiac dysfunction and heart failure [39,40]. Phosphorylation mediated by casein kinase 1 delta/epsilon (CK1δ/ε) promotes nuclear translocation and degradation of PER1/2 [12,41], which is critical for the clock period [8**,42*,43]. Mutation in CK1ε (tau mutant) that impairs the kinase activity reduces the amplitude of Bmal1 circadian expression and induces a phase shift for Per1 and Dbp in hamster heart [43]. In addition, Durgan et al. showed that Bmal1 is O-GlcNAcylated in mouse heart with the peak O-GlcNAcylation level in the middle of the active phase [44**]. O-GlcNAcylation of BMAL1/CLOCK inhibits their ubiquitination and promotes the transcriptional activity [45,46]. As the O-GlcNacylation modification is sensitive to glucose level, future study may investigate whether O-GlcNacylation can serve as a metabolic sensor to regulate circadian clock in heart [47*].

2.3. The core clock affects cardiac function through both transcription and translation

2.3.1. The core clock regulates circadian transcription

Approximately 10% of the gene transcription oscillates in the heart similar to other organs in the body [16,17,20,48,49]. In addition to acetylating BMAL1, CLOCK also functions as a histone acetyltransferase (HAT) for histone H3 [50]. BMAL1 enhances the HAT function and serves as pioneer factor for driving the circadian transcription due to its accessibility to closed chromatin [51,52]. A comprehensive interrogation of the cistrome profile uncovered the circadian rhythmic recruitments of BMAL1 and RNA polymerase II (RNAPII) that occur on a genome-wide scale [53,54]. This landmark finding suggests that global mobilization of BMAL1 and RNAPII is the foundation for circadian oscillation in gene transcription and cardiac function.

2.3.2. The core clock regulates circadian translation

The activity of protein synthesis also oscillates in a circadian fashion in the heart [55]. CBK mouse hearts have increased translation level [55]. Detailed biochemical study showed that BMAL1 serves as a negative regulator for AKT signaling in the heart [55] which activates the mammalian target of rapamycin (mTOR)/ribosomal protein S6/eukaryotic translation initiation factor 4E-binding protein 1 signaling axis, a classic mechanism that promotes protein synthesis [56]. Importantly, loss of BMAL1 persistently activates mTOR signaling leading to increased protein synthesis level as well as attenuated autophagy, which causes cardiac hypertrophy [55].

3. The slave clock in heart

While each tissue has about 5-10% of the genes oscillating, the overlap between different tissues is exceedingly small, suggesting a local factor interfacing with the core clock and determines the tissue specificity of transcript oscillation [48,57]. The concept of “slave clocks” has long been proposed to explain this phenomenon, however, there is very few examples of well described slave clocks. Our work demonstrated that the Kruppel-like factor 15 (KLF15) is an essential slave clock in the heart (Figure 2A) [58]. KLF15 is a transcription factor that oscillates in a circadian fashion under the control of BMAL1 [59]. Additionally, a recent study identified enhancer of zeste homolog 2 (EZH2) as a novel suppressor for Klf15 transcription in the heart in the setting of ischemic heart failure [60**]. EZH2 may also modulates the circadian function of KLF15 as it directly associates with CLOCK:BMAL1 [61], the details of which, remain to be studied.

Figure 2. Cardiac slave clock KLF15.

(A) Circadian regulation of KLF15 and its slave clock function in both gene activation and repression. (B) KLF15 coordinates catabolism and ROS clearance in the active phase by regulation of myocardial NAD⁺ level. (C) The dual regulation of BMAL1 and KLF15 on cardiac NAD⁺ biosynthesis. Both BMAL1 and KLF15 activates Nampt transcription through direct binding on the promoter/enhancer. The increased NAD⁺ level activates SIRT1 activity, which deacetylates BMAL1 and histone H3 at the cis-acting site and suppresses the transactivation of BMAL1, forming a feedback inhibition loop. Additionally, KLF15 is subjected to the regulation of various metabolic inputs, which fine-tunes the circadian oscillatory expression of Nampt to achieve precise control of cardiac NAD⁺ level.

Circadian RNA-seq revealed that KLF15 governs 75% of oscillating genes in the heart without affecting the oscillation of the core clock, including direct regulation of catabolic genes in the active phase [58]. Additionally, KLF15 allows the recruitment of REV-ERB and nuclear receptor corepressor which suppresses hundreds of genes from aberrant oscillation [58]. KLF15 deficiency leads to cardiac lipid utilization defect and cardiomyopathy [58,62*]. Further, our recent work demonstrated that KLF15 also regulates MnSOD activity through acetylation on lysine 122 in a SIRT3-dependent fashion and provides circadian cardioprotection against oxidative stress induced by I/R injury [63**]. Thus, KLF15 coordinates the catabolic activity and clearance of the accompanied oxidative stresses in a circadian fashion, thereby exemplifying one of the proposed evolutionary benefit of circadian rhythm (Figure 2B). In addition to the core clock, KLF15 is also regulated by other physiological and pathological inputs such as diet and exercise, disease status and aging. For instance, Prosdocimo et al. showed that KLF15 expression in the mouse heart may be induced by fasting, and activates lipid metabolic genes and promotes myocardial lipid utilization [62*]. Thus in contrast to what’s implied by their name, “slave clocks” are not completely subordinate to the core clock, but integrate circadian rhythmic gene expression with local lineage specific information as well as metabolic signals (Figure 2C).

4. Effectors for circadian rhythm in cardiac function

Ultimately, the diverse rhythmic activity of the heart is carried out by various effectors, such as ion channels, metabolic enzymes, and signaling molecules. Here, we will review the major categories of effectors with implications in cardiac function (Table 1). The effectors that have exhibited circadian rhythm in in vivo studies will be discussed here in more details.

Table 1.

Effectors for circadian rhythm in cardiac function.

Category	Effector	Circadian regulation on cardiac physiology & disease susceptibility
Ion channel	SCN5A, CACNA1C, KChIP2	Heart rate and cardiac electrophysiology (repolarization); arrhythmia
Metabolic enzyme	NAMPT	NAD+ biosynthesis
	DGCT2	Triacylglycerol biosynthesis
	BDH1	Ketone body catabolism
Metabolite	NAD+	Cardiac metabolism and SIRT1/3-dependent regulation; I/R injury
	H2O2	Redox cycling of Prx III and mitochondrial function
Intracellular signaling	AMPK	Cardiac metabolism (NAMPT and hormone-sensitive lipase)
	PI3K/AKT (mTOR & GSK3β)	Translation, autophagy, hormone (insulin) response and Cav1.2 expression; hypertrophy and I/R injury
	PKA	β-adrenergic signaling, SERCA2 activity and contractility
	CaN	β-adrenergic signaling and CaN signaling; I/R injury and heart failure
MicroRNA	miR-21	Glycolysis and cardiomyocyte survival; I/R injury
Novel mechanism	?	Mitochondrial dynamics (mitophagy, fission and fusion), alternative splicing, long non-coding RNA and BMAL1-independent clock

4.1. Ion channels

Ion channels are the molecular basis for cardiac electrophysiology [64]. Comparing to neurological tissues, cardiac ion channels have shorter half-lives (hours) and undergo more rapid turnover [65,66]. Therefore, by modulating the expression of cardiac ion channel genes (e.g. Scn5a, Kcnip2 and Cacna1c) the circadian clock readily regulates cardiac electrophysiology in a rhythmic fashion [19,59,67**,68]. For instance, BMAL1 and KLF15 directly regulates the rhythmic expression of Nav1.5 channel (SCN5A) [19,59] and Kv channel interacting protein 2 (KChIP2) [59], respectively. Studies from CCM, CBK and KLF15-null models indicate that loss of circadian clock can cause abnormal cardiac electrophysiology through effector channels and result in slowed heart rate, prolonged QRS intervals [19], reduced rhythmic amplitude of heart rate [16] and absence of oscillation in QT variation [59], which increases the risk for arrhythmia.

4.2. Metabolic enzymes

The heart demands continuous energy to support its contractile function [69,70]. Cardiac metabolism is tightly controlled by metabolic enzymes where dysregulation may lead to energy failure, metabolic remodeling and heart failure [71,72,73]. Early expression analysis of CBK mice revealed 19 putative direct BMAL1 target genes including three genes encoding essential metabolic enzymes in the heart [20], which are Nampt for nicotinamide adenine dinucleotide (NAD⁺) biosynthesis [63**,74,75,76**,77], Dgat2 for triacylglycerol biosynthesis [78,79,80,81*], and Bdh1 for ketone body catabolism [82].

Circadian rhythm in metabolic enzyme highly correlates with the metabolic status in the heart. For instance, nicotinamide phosphoribosyltransferase (NAMPT) level is increased prior to the sleep-to-wake transition in the heart in anticipation of the upcoming active phase when the demand of NAD⁺ will be elevated for fuel catabolism [20,63**,77,83]. Mechanistically, circadian transcription of Nampt is controlled by the dual regulation of BMAL1 (core clock) [36,37,84,85,86,87] and KLF15 (slave clock) [63**], in which KLF15 integrates external metabolic inputs [88,89**,90,91] and fine-tunes NAD⁺ biosynthesis besides the clock control (Figure 2C). KLF15 also transcriptionally activates numerous other effector enzymes in the fatty acid and multiple amino acid metabolic pathways [58]. Dysregulation of these effectors has been associated with I/R injury [63**,74,80] and heart failure [62*,74,75,76**,78,92].

4.3. Metabolites

In addition to providing building blocks and energy, metabolites serve important signaling functions and connects cardiac metabolism with contractile function and cell survival in the heart [93,94].

4.3.1. NAD⁺

The NAD⁺ level in the heart is higher than most of the other organs [95]. Besides interacting with over 500 enzymes, NAD⁺ is involved in almost every key biological process in the mammalian cells [96,97**]. Dysregulated NAD⁺ homeostasis has been associated with various CVD and NAD⁺-boosting is emerging as a novel strategy for treating and preventing CVD [97**, 98*,99,100**].

The oscillatory NAD⁺ level in heart is primarily governed by the circadian NAMPT levels [20,97**,77,83]. This is likely further driving the rhythmic activity of certain NAD⁺-dependent enzymes that have K_m values within the physiological range of NAD⁺(300-700 μM) [97**,101], such as SIRT1 (94-888 μM) [102,103,104,105] and SIRT3 (280-880 μM) [106,107]. Indeed, we found that KLF15 regulates circadian susceptibility to I/R injury through the KLF15-Nampt-NAD⁺-SIRT3-MnSOD axis [63**]. Further, we found administration of an NAD⁺ precursor, nicotinamide mononucleotide (NMN) provided cardioprotection against I/R injury when the NAD⁺ level was at nadir but not when the NAD⁺ level was at peak, supporting the use of chronotherapy in NAD⁺-boosting strategies [63**].

4.3.2. H₂O₂

In mammalian cells, most of the H₂O₂ is produced in the mitochondria [108]. Excessive production of H₂O₂ can be detrimental to cardiac function [109**] which underlies a number of CVD [110,111,112,113]. Peroxiredoxin III (Prx III) is the major H₂O₂-eliminating enzyme in the mitochondria where it reduces H₂O₂ into H₂O in the presence of thioredoxin (Trx) through the catalytic cycle [114,115,116,117]. However, when H₂O₂ production rate is boosted due to increased energy demand during the active phase, Prx III can undergo hyperoxidization cycle, through which it turns into its inactive form (PrxIII-SO₂) [114,117]. A redox cycling between Prx III and PrxIII-SO₂ is then established based on H₂O₂ release from the mitochondria and mitochondrial translocation of sulfiredoxin (Srx) that reactivates Prx III (Figure 3A) [117,118]. This mechanism was found to exhibit a circadian rhythm in many types of cells/tissues including the heart [118,119,120,121,122,123]. H₂O₂ could potentially damage the mitochondrial [124] and the cardiac function [109**], however, a direct investigation of mitochondrial function correlating with oscillating H₂O₂ level in a circadian fashion has not be performed.

Figure 3. Effectors of cardiac circadian clock.

(A) Circadian H₂O₂ release and Prx III redox cycling. During the resting phase, mitochondrial H₂O₂ level is low, Prx III is able to scavenge all the mitochondrial H₂O₂ through catalytic cycle. While in the active phase, the H₂O₂ production is increased due to increased cardiac metabolism, which lead to hyperoxidization of Prx III and the formation of the inactive Prx III-SO₂. Excessive H₂O₂ released into cytosol oxidizes Srx leading to the formation of the Srx:HSP30 complex, which is required to reactivate Prx III.

(B) Circadian regulation of PKA/β-adrenergic and CaN signaling, resulting in low diastolic intracellular Ca²⁺ concentration ([Ca²⁺]_i) in the active phase which favors increased contractility, and high diastolic [Ca²⁺]_I in the resting phase which favors hypertrophy.

4.4. Intracellular signaling pathways

4.4.1. AMPK signaling

AMPK is a metabolic sensor that integrates the energy status with circadian regulation on key metabolic pathways [125,126]. For example, AMPK regulates the rhythmic activity of NAMPT [77] and hormone-sensitive lipase [127] which affects the NAD⁺ metabolism and the triglyceride metabolism in heart, respectively. The core clock regulation of AMPK is evidenced by the CCM model where oscillation in AMPK activity is lost [127]. Notably, CCM only disrupts the rhythm without affecting the baseline expression [127]. On the other hand, AMPK can also influence the core clock activity by phosphorylation on core clock suppressor CRY1/2 [128] and PER2 [129] which promotes their degradation.

4.4.2. PI3K/AKT (mTOR and GSK3β) signaling

PI3K/AKT is involved in diverse biological processes such as cell proliferation, survival and migration [130,131]. PI3K/AKT signaling activates mTOR but inhibits GSK3β [131,132]. The core clock controls the circadian rhythm in PI3K/AKT signaling as well as mTOR and GSK3β. Expression analysis of CBK mice revealed that BMAL1 directly regulates Pik3r1 which encodes p85α, the regulatory subunit of PI3K [20]. Consistently, CCM or CBK heart exhibits disrupted circadian rhythm in p85α expression and also the phosphorylation states of AKT, mTOR and GSK3β [55,133]. The importance of circadian PI3K/AKT signaling in cardiac function has been demonstrated in vivo. McGinnis et al. showed that loss of circadian PI3K signaling in CBK mouse disrupted circadian mTOR signaling, which led to increased protein synthesis rate, suppressed autophagy and cardiac hypertrophy [55]. Additionally, PI3K/AKT signaling has been shown to contribute to the oscillatory physiological response to insulin [55] and the circadian rhythmic expression of Ca_v1.2 channel (CACNA1C) [68].

4.4.3. PKA signaling

PKA signaling transduces β-adrenergic signaling and phosphorylates several proteins that are essential for Ca²⁺ transient such as L-type calcium channel (Ca_v1.2 [134] and β_2a subunits [135]), ryanodine receptor 2 (RYR2) [136], and phospholamban (PLN) [137], which ultimately increases the contractile function in heart (inotropic) (Figure 3B) [138,139,140,141]. One study found modest transcriptional oscillation of Prkar1a, a gene encodes the type 1a regulatory subunit of PKA [142], which was disrupted in the CCM heart [16]. Both of the Prkar1a transcriptional level [16] and the circulating β-agonist level exhibit in-phase rhythm with peak during the active phase [143]. Therefore, although lacking direct experimental data, it is likely that the actual activity of the PKA signaling in the heart also oscillates in a similar pattern, which may contribute to circadian Ca²⁺ transient, sarcoplasmic/endoplasmic reticulum Ca²⁺ ATPase 2 (SECRCA2) activity and contractility in heart [8**,144,145].

4.4.4. Ca²⁺ and calcineurin signaling

The calcium-activated protein phosphatase or CaN links the Ca²⁺ signaling and protein phosphorylation, which is particularly important for excitatory cells such as the cardiomyocytes [146]. CaN also regulates mitochondrial dynamics [146,147,148,149,150], which is associated with I/R injury [151**], cardiomyocyte hypertrophy [152] and heart failure [147]. Activity of CaN exhibits circadian rhythm in the heart [144]. The oscillatory transcription of an important CaN-targeted gene, Rcan1 (regulator of CaN 1) is diminished in the CCM heart, suggesting that circadian CaN activity requires the core clock [16]. Circadian rhythm in CaN is influenced by two interconnected mechanisms (Figure 3B) [144,153]. First, CaN responses to cytosolic Ca²⁺ level change [146] which is influenced by the aforementioned circadian β-adrenergic/PKA signaling cascade [144]. Second, the expression of RCAN1 isoform 4 (RCAN1.4 with exon 4 being the first exon [154,155]), a CaN inhibitor [156,157], is circadian in the heart [144]. The rhythmic RCAN1.4 level is partially regulated by CaN-triggered nuclear translocation of nuclear factor of activated T cells (NFAT) [144,158,159]. However, CaN-independent mechanism also exists as inhibition of CaN activity only reduces the peak transcription of Rcan1.4 but does not abolish its rhythmicity in the heart [144].

Circadian CaN activity modulates the functional consequence of β-agonist/PKA signaling in a daily fashion (Figure 3B). CaN activity reaches trough at the sleep-to-wake transition so that key modulators for β-adrenergic signaling such as PLN [160,161] could maintain in the phosphorylated state (inactive). This helps maximize the stimulation of contractile function by β adrenergic signaling and helps anticipate the elevation of cardiac workload in the upcoming active phase [144]. Indeed, epinephrine induces a greater increase in cardiac power ex vivo during the active phase than inactive phase [16]. On the other hand, isoproterenol induces a more pronounced hypertrophic growth at the wake-to-sleep transition, [162] likely due to high CaN activity that promotes cardiac hypertrophy [163]. Moreover, high CaN activity increases dephosphorylation of PLN and impairs Ca²⁺ uptake by SERCA2 which elevates diastolic Ca²⁺ level [140,164,165,166] and may further enhance the CaN-induced hypertrophy.

4.5. MicroRNAs

Several oscillatory miRNAs have been implicated for cardiac function, however, the molecular mechanism remains sparse [167,168]. Recently, Bartman et al. demonstrated that miR-21 is a downstream target of PER2 that exhibits rhythmic expression in the heart. They showed that miR-21 plays a crucial role in glycolysis, metabolic adaptation and cardioprotection in response to I/R injury [22,169]. Intense light exposure induces PER2 expression [170,171] and upregulates miR-21 level in mouse heart and human plasma [169], suggesting a potential therapeutic benefit for intense light therapy through this pathway [167]. Future study is required to elucidate the detailed mechanisms of circadian miRNAs in various diseases settings.

5. Novel mechanisms

Recent evidence suggests that circadian rhythm also exist in mitochondrial dynamics (mitophagy, fission and fusion) [8**,172,173,174,175], alternative splicing [176,177**] and long non-coding RNA [178] in the heart. In addition, BMAL1-independent clock that has been found in other types of cells/tissues [119,120,121,122,179**,180,181,182,183**] may also play a role in the heart. Future studies exploring these novel mechanisms will elucidate exciting new avenues in the circadian regulation of the cardiac function.

6. Conclusion

In the modern society exposures to artificial lighting and temperature control disrupt our intrinsic circadian rhythm. Approximately 15%-20% of industrial workers are shift workers who have increased risks for hypertension and coronary artery disease [184,185,186,187,188,189]. The contribution of circadian disruption to CVD inevitably increases with continued modernization. Thus, understanding the molecular mechanisms and designing novel therapeutic strategies targeting the core clock or the slave clocks as well as chronotherapies targeting the effectors are in urgent need for the treatment and prevention of CVD.

The core clock-slave clock-effectors forms an expansive and intricate system to optimize the physiological function of the heart. We have just begun to understand this sophisticated system. The discovery of KLF15 as an essential slave clock that coordinate s intrinsic core clock and metabolic output has open up a new dimension in the cardiac circadian rhythm research. Although KLF15 regulates about 75% of the oscillatory gene expression in heart, it still remains an open question whether there’s other slave clock in the heart. In addition, it is not well understood how external metabolic signals regulate KLF15. Future study may also explore the roles of novel effectors in regulating circadian rhythm in heart (Table 5), particularly through non-transcriptional mechanisms, such as microRNA, long non-coding RNA and alternative splicing, all of which can potentially lead to broad regulation of gene program and cellular function.

Highlights.

• A network of core clock-slave clock-effectors links the molecular clock to cardiac function

• The core clock machinery BMAL1:CLOCK regulates circadian rhythm through both transcription and translation in the heart

• KLF15 is an essential slave clock in the heart that coordinates cardiac catabolism and clearance of the accompanied oxidative stresses in a circadian fashion

• Circadian rhythm in cardiac function is carried out by various circadian effectors including ion channel, metabolic enzyme, metabolite, intracellular signaling, microRNA as well as other novel mechanisms.

• Disruption of intrinsic circadian rhythm is highly associated with cardiovascular disease