Sudden cardiac death caused by ventricular arrhythmias is the principal cause of mortality from heart disease in the world: its occurrence increases in first hours of the morning between 6 and 10 a.m. with a second peak in the evening hours.
The incidence of sudden cardiac death exhibits diurnal variation in both acquired and hereditary forms of heart disease.
A common mechanism in both forms that increases susceptibility to ventricular arrhythmias is abnormal myocardial repolarization.Clinically, three common types of alterations in myocardial repolarization are evident on the ECG:
- Prolongation of repolarization (e.g. heart failure, long QT syndrome);
- Shortening of repolarization (e.g. short QT syndrome);
- Early repolarization (e.g. Brugada syndrome).
All three modifications of repolarization increase vulnerability to ventricular arrhythmias.
Several physiological parameters in the cardiovascular system such as heart rate, blood pressure, vascular tone, QT interval and ventricular effective refractory period show diurnal variation.
Specifically, it has been proved that cardiac ion channels expression and also QT interval duration (an index of myocardial repolarization) show an endogenous circadian rhythmicity under control of a circadian oscillator, Krüppel-like factor 15 (Klf15).
Structure of Klf 15
Klf15 is a protein encoded in humans by the KLF15 gene and it’s part of the Krüppel-like factors (KLF) family.
The KLF family is a group of transcription factors which regulate several cell processes including proliferation, differentiation, growth, development, survival and responses to exogenous stress.
KLFs are implied in the regulation of cardiovascular, digestive, respiratory, hematological and immune systems and are involved in disorders such as obesity, cardiovascular disease, cancer and inflammatory syndromes.
In the mammals there are 17 genes encoding for the KLF family. Each KLF has a specific set of three DNA-binding zinc finger domains at its C-terminus; each one recognizes 3 base pairs in DNA. DNA binding sites include GC-rich sequences.
However, the KLFs N-terminus functional domains vary between family members, allowing them to bind different co-activators, co-repressors and modifiers, generating functional KLFs diversity and specificity.
Expression of Klf15 - Tissue specificity and function
KLF15 is highly expressed in:
- Liver : KLF15 is involved in metabolism, specifically in the positive regulation of gluconeogenesis.
- Skeletal muscle : KLF15 levels control the muscle ability to burn fat and generate energy in response to exercise.
- Adipocytes : KLF15 plays an essential role in regulating adipocyte differentiation through its regulation of PPARγ expression; it is also involved in adipogenesis.
- Cardiomyocytes :
- KLF15 inhibits cardiac hypertrophy by attenuating the functions of the transcription factors GATA-4 and myocyte enhancer factor 2 (MEF2) — two effectors of cardiomyocytes hypertrophy. Moreover, a KLF15 overexpression in cadiomyocytes reduces cell size and also the expression of atrial (aNP) and brain (bNP) natriuretic factors, both associated with cardiac hypertrophy.
- KLF15 also protects from an excessive deposition of connective tissue in the heart, by suppressing connective tissue growth factor (CTGF) expression, which is upregulated by transforming growth factor beta (TGF-beta1) during cardiac fibrosis.
- Moreover, KLF15 binds the promoter of Kv channel-interacting protein 2 (KCNIP2) and it regulates KCNIP2 circadian expression in the heart.
Mammalian Kruppel-Like Factors in Health and Diseases - PubMed,2010
Klf15 expression exhibits 24 hour rhythmicity
The core clock machinery (CCM) is the hypothalamus circadian pacemaker, situated in the suprachiasmatic nucleus and also in most peripheral cells. The hypothalamic CCM is activated by several triggers (e.g. light exposure) and acts via neural and endrocrine signals to synchronize the CCM in peripheral tissues. The signalation pathway consists of positive and negative feedback loops: those loops are generated by transcription factors as Clock and Bmal1.
It has been reported that in hepatocytes the regulatory region of Klf15s shows four “E-box” motifs, periodically binded by Clock and Bmal1: this mechanism induces the transcription of Klf15 in a dose dependent manner. Indeed, the rhythmic occupancy of Bmal1 and Clock on the Klf15 promoter supports a direct role for the CCM in regulating the 24 hour-periodicity of Klf15.
Circadian rhythms govern cardiac repolarization and arrhythmogenesis
By exposing mice models to a periodical light/dark switch, some studies showed that the cardiomyocyte expression of the structural alpha-subunit (Kv4.2) and the regulatory beta-subunit (KChIP2) of the transient outward potassium current (I to) exhibit a circadian rhythmicity, as well as the QTc interval.
(in figura Kcnd2=Kv4.2)
Indeed, Kv4.2 expression exhibited altered rhythmic variation in Klf15-knock out (k.o.) mice and KChIP2 expression showed no rhythm in the Klf15-k.o. mice. These observations demonstrate that, depending on the endogenous clock, Klf15 regulates rhythmic changes in KChIP2 and Kv4.2 cardiac expression. In support of these results it has been reported that the KChIP2 promoter region has several KLF-binding sites. These data suggest that KChIP2 is a direct transcriptional target for Klf15 in the heart. Furthermore, the expression of Klf15 and KChIP2 was altered in a similar way in Bmal1-k.o. mice.
(in figura Kcnd2=Kv4.2)
Circadian variation of cardiac K+ Channel gene expression - Circulation,2003
These studies also examined if the Klf15-dependent regulation of KChIP2 could be responsible for rhythmic day/night variation in myocardial repolarization:
- In Klf15-k.o. mice, the ECG QTc interval was prolonged during the dark phase and failed to oscillate;
- In Klf15-gain of function mice, the ECG shows both a short QT interval with no rhythmic light/dark variation and also ST-segment changes suggestive of early-repolarization.
(in figura Klf15-null=Klf15-k.o.;Klf15-Tg=Klf15-gain of function)
Authors then isolated cardiomyocytes to see if I to -dependent changes in cardiac repolarization were possibly responsible for the ECG changes:
- In Klf15-k.o. mice there was a marked reduction in I to density and a prolongation of action potential duration (APD);
- In Klf15-gain of function mice there was a significant increase in I to density with a shortening of APD.
These data suggest that Klf15-dependent transcriptional regulation of rhythmic KChIP2 expression in murine hearts plays a central role in rhythmic variation of the ventricular repolarization.
These studies argued if the excessive prolongation or shortening of cardiomyocytes repolarization could determine arrhythmia susceptibility.
- Klf15-k.o mice exhibit an increased occurrence of ventricular arrhythmias. These mice showed no evidence of ventricular dysfunction or fibrosis, so the enhanced susceptibility to arrhythmias is probably due to abnormalities in repolarization;
- Klf15-gain of function mice exhibit spontaneous ventricular arrhythmias on ECG.
In conclusion, these studies demonstrate that both deficiency and excess of Klf15 are involved in temporal variation of cardiac repolarization and also in increased susceptibility to arrhythmias.
(in figura Klf15-null=Klf15-k.o.;Klf15-Tg=Klf15-gain of function)
The circadian control of KChIP2 by Klf15 establishes the principle that circadian rhythms may contribute to arrhythmogenesis. Moreover,it was noted that Klf15 minimally affects Kv4.2 expression, which also exhibits a circadian rhythm. However, Kv4.2 expression was disrupted in Bmal1-k.o. mice hearts and this is suggestive of a direct regulation by the CCM. According to this observation, cardiomyocytes from Bmal1-k.o. mice exhibit APD prolongation due to near complete elimination of the fast component of I to. This could support the hypothesis that additional factors may impact on temporal variation in arrhythmogenesis.
Circadian rhythms govern cardiac repolarization and arrhythmogenesis - PubMed,2012
Conclusions
In summary, there’s a link between endogenous circadian rhythms and cardiac electrical instability, often associated with sudden cardiac death in humans. Specifically, the Klf15-dependent rhythmic transcription of KChIP2 regulates the duration and the pattern of cardiomyocytes repolarization, hence the susceptibility to arrhythmias in mice. Since the occurrence of sudden cardiac death shows a diurnal pattern, these observations offer new points of view about possible triggers of electrical instability in the heart. However, in contrast to murine repolarization (largely dependent on I to), human repolarization occurs through multiple ionic channels.
Additional studies will be needed to understand the link between the circadian clock and the electrophysiological properties of the human heart, in order to prevent and treat arrhythmias by modulating the CCM.
