Circadian rhythm in QT interval is preserved in mice deficient of potassium channel interacting protein 2

When the Clock Strikes A-fib

Within the broad spectrum of atrial fibrillation (AF) symptomatology, there is a striking subset of patients with predominant or even solitary nocturnal onset of the arrhythmia. This review covers AF with nocturnal onset, with the aim of defining this distinctive subgroup among patients with AF. A periodicity analysis is provided showing a clear increased onset between 10:00 pm and 7:00 am. Multiple interacting mechanisms are discussed, such as circadian modulation of electrophysiological properties, vagal tone, and sleep disorders, as well as the potential interaction and synergism between these factors, to provide a better understanding of this clinical entity. Lastly, potential therapeutic targets for AF with nocturnal onset are addressed such as upstream therapy for underlying comorbidities, type of drug and timing of drug administration and pulmonary vein isolation, ablation of the ganglionated plexus, and autonomic nervous system modulation. Understanding the underlying AF mechanisms in the individual patient with nocturnal onset will contribute to patient-specific therapy.

Cardiac GR Mediates the Diurnal Rhythm in Ventricular Arrhythmia Susceptibility

BACKGROUND

Ventricular arrhythmias (VAs) demonstrate a prominent day-night rhythm, commonly presenting in the morning. Transcriptional rhythms in cardiac ion channels accompany this phenomenon, but their role in the morning vulnerability to VAs and the underlying mechanisms are not understood. We investigated the recruitment of transcription factors that underpins transcriptional rhythms in ion channels and assessed whether this mechanism was pertinent to the heart's intrinsic diurnal susceptibility to VA.

METHODS AND RESULTS

Assay for transposase-accessible chromatin with sequencing performed in mouse ventricular myocyte nuclei at the beginning of the animals' inactive (ZT0) and active (ZT12) periods revealed differentially accessible chromatin sites annotating to rhythmically transcribed ion channels and distinct transcription factor binding motifs in these regions. Notably, motif enrichment for the glucocorticoid receptor (GR; transcriptional effector of corticosteroid signaling) in open chromatin profiles at ZT12 was observed, in line with the well-recognized ZT12 peak in circulating corticosteroids. Molecular, electrophysiological, and in silico biophysically-detailed modeling approaches demonstrated GR-mediated transcriptional control of ion channels (including Scn5a underlying the cardiac Na+ current, Kcnh2 underlying the rapid delayed rectifier K+ current, and Gja1 responsible for electrical coupling) and their contribution to the day-night rhythm in the vulnerability to VA. Strikingly, both pharmacological block of GR and cardiomyocyte-specific genetic knockout of GR blunted or abolished ion channel expression rhythms and abolished the ZT12 susceptibility to pacing-induced VA in isolated hearts.

CONCLUSIONS

Our study registers a day-night rhythm in chromatin accessibility that accompanies diurnal cycles in ventricular myocytes. Our approaches directly implicate the cardiac GR in the myocyte excitability rhythm and mechanistically link the ZT12 surge in glucocorticoids to intrinsic VA propensity at this time.

Preload dependence in an animal model of mild heart failure with preserved ejection fraction (HFpEF)

AIM

Heart Failure with preserved Ejection Fraction (HFpEF) is characterized by diastolic dysfunction and reduced cardiac output, but its pathophysiology remains poorly understood. Animal models of HFpEF are challenging due to difficulties in assessing the degree of heart failure in small animals. This study aimed at inducing HFpEF in a mouse model to probe preload-dependency.

METHODS

Increased body mass and arterial hypertension were induced in mice using a Western diet and NO synthase inhibition. Preload dependence was tested ex vivo.

RESULTS

Mice with obesity and hypertension exhibited reduced cardiac output, indicating a failing heart. Increased left ventricular filling pressure during diastole suggested reduced compliance. Notably, the ejection fraction was preserved, suggesting the development of HFpEF. Spontaneous physical activity at night was reduced in HFpEF mice, indicating exercise intolerance; however, the cardiac connective tissue content was comparable between HFpEF and control mice. The HFpEF mice showed increased vulnerability to reduced preload ex vivo, indicating that elevated left ventricular filling pressure compensated for the rigid left ventricle, preventing a critical decrease in cardiac output.

CONCLUSION

This animal model successfully developed mild HFpEF with a reduced pump function that was dependent on a high preload. A model of mild HFpEF may serve as a valuable tool for studying disease progression and interventions aimed at delaying or reversing symptom advancement, considering the slow development of HFpEF in patients.

Circadian regulation of Ca V 1.2 expression by RORα in the mouse heart

Background

In addition to show autonomous beating rhythmicity, the physiological functions of the heart present daily periodic oscillations. Notably the ventricular repolarization itself varies throughout the circadian cycle which was mainly related to the periodic expression of K + channels. However, the involvement of the L-type Ca 2+ channel (Ca V 1.2 encoded by Cacna1c gene) in these circadian variations remains elusive.

Methods

We used a transgenic mouse model (PCa-luc) that expresses the luciferase reporter under the control of the cardiac Cacna1c promoter and analyzed promoter activity by bioluminescent imaging, qPCR, immunoblot, Chromatin immunoprecipitation assay (ChIP) and Ca V 1.2 activity.

Results

Under normal 12:12h light-dark cycle, we observed in vivo a biphasic diurnal variation of promoter activities peaking at 9 and 19.5 Zeitgeber time (ZT). This was associated with a periodicity of Cacna1c mRNA levels preceding 24-h oscillations of Ca V 1.2 protein levels in ventricle (with a 1.5 h phase shift) but not in atrial heart tissues. The periodicity of promoter activities and Ca V 1.2 proteins, which correlated with biphasic oscillations of L-type Ca 2+ current conductance, persisted in isolated ventricular cardiomyocytes from PCa-Luc mice over the course of the 24-h cycle, suggesting an endogenous cardiac circadian regulation. Comparison of 24-h temporal patterns of clock gene expressions in ventricles and atrial tissues of the same mice revealed conserved circadian oscillations of the core clock genes except for the retinoid-related orphan receptor α gene (RORα), which remained constant throughout the course of a day in atrial tissues. In vitro we found that RORα is recruited to two specific regions on the Cacna1c promoter and that incubation with specific RORα inhibitor disrupted 24-h oscillations of ventricular promoter activities and Ca V 1.2 protein levels. Similar results were observed for pore forming subunits of the K + transient outward currents, K V 4.2 and K V 4.3.

Conclusions

These findings raise the possibility that the RORα-dependent rhythmic regulation of cardiac Ca V 1.2 and K V 4.2/4.3 throughout the daily cycle may play an important role in physiopathology of heart function.

Prolonged QT intervals in mice with cardiomyocyte-specific deficiency of the molecular clock

AIM

Cardiac arrhythmias and sudden deaths have diurnal rhythms in humans. The underlying mechanisms are unknown. Mice with cardiomyocyte-specific disruption of the molecular clock genes have lower heart rate than control. Because changes in the QT interval on the electrocardiogram is a clinically used marker of risk of arrhythmias, we sought to test if the biological rhythms of QT intervals are dependent on heart rate and if this dependency is changed when the molecular clock is disrupted.

METHODS

We implanted radio transmitters in male mice with cardiomyocyte-specific Bmal1 knockout (CBK) and in control mice and recorded 24-h ECGs under diurnal and circadian conditions. We obtained left ventricular monophasic action potentials during pacing in hearts ex vivo.

RESULTS

Both RR and QT intervals were longer in conscious CBK than control mice (RR: 117 ± 7 vs 110 ± 9 ms, P < .05; and QT: 53 ± 4 vs 48 ± 2 ms, P < .05). The prolonged QT interval was independent of the slow heart rate in CBK mice. The QT interval exhibited diurnal and circadian rhythms in both CBK and control mice. The action potential duration was longer in CBK than in control mice, indicating slower repolarization. Action potential alternans occurred at lower pacing rate in hearts from CBK than control mice (12 ± 3 vs 16 ± 2 Hz, respectively, P < .05).

CONCLUSION

The bradycardic CBK mice have prolonged ventricular repolarization independent of the heart rate. Diurnal and circadian rhythms in repolarization are preserved in CBK mice and are not a consequence of the 24-h rhythm in heart rate. Arrhythmia vulnerability appears to be increased when the cardiac clock is disrupted.

Circadian Rhythms of Early Afterdepolarizations and Ventricular Arrhythmias in a Cardiomyocyte Model

Sudden cardiac arrest is a malfunction of the heart's electrical system, typically caused by ventricular arrhythmias, that can lead to sudden cardiac death (SCD) within minutes. Epidemiological studies have shown that SCD and ventricular arrhythmias are more likely to occur in the morning than in the evening, and laboratory studies indicate that these daily rhythms in adverse cardiovascular events are at least partially under the control of the endogenous circadian timekeeping system. However, the biophysical mechanisms linking molecular circadian clocks to cardiac arrhythmogenesis are not fully understood. Recent experiments have shown that L-type calcium channels exhibit circadian rhythms in both expression and function in guinea pig ventricular cardiomyocytes. We developed an electrophysiological model of these cells to simulate the effect of circadian variation in L-type calcium conductance. In our simulations, we found that there is a circadian pattern in the occurrence of early afterdepolarizations (EADs), which are abnormal depolarizations during the repolarization phase of a cardiac action potential that can trigger fatal ventricular arrhythmias. Specifically, the model produces EADs in the morning, but not at other times of day. We show that the model exhibits a codimension-2 Takens-Bogdanov bifurcation that serves as an organizing center for different types of EAD dynamics. We also simulated a two-dimensional spatial version of this model across a circadian cycle. We found that there is a circadian pattern in the breakup of spiral waves, which represents ventricular fibrillation in cardiac tissue. Specifically, the model produces spiral wave breakup in the morning, but not in the evening. Our computational study is the first, to our knowledge, to propose a link between circadian rhythms and EAD formation and suggests that the efficacy of drugs targeting EAD-mediated arrhythmias may depend on the time of day that they are administered.

Aging Disrupts Normal Time-of-Day Variation in Cardiac Electrophysiology

BACKGROUND

Cardiac gene expression and arrhythmia occurrence have time-of-day variation; however, daily changes in cardiac electrophysiology, arrhythmia susceptibility, and Ca²⁺ handling have not been characterized. Furthermore, how these patterns change with age is unknown.

METHODS

Hearts were isolated during the light (zeitgeber time [ZT] 4 and ZT9) and dark cycle (ZT14 and ZT21) from adult (12-18 weeks) male mice. Hearts from aged (18-20 months) male mice were isolated at ZT4 and ZT14. All hearts were Langendorff-perfused for optical mapping with voltage- and Ca²⁺-sensitive dyes (n=4-7/group). Cardiac gene and protein expression were assessed with real-time polymerase chain reaction (n=4-6/group) and Western blot (n=3-4/group).

RESULTS

Adult hearts had the shortest action potential duration (APD) and Ca²⁺ transient duration (CaTD) at ZT14 (APD₈₀: ZT4: 45.4±4.1 ms; ZT9: 45.1±8.6 ms; ZT14: 34.7±4.2 ms; ZT21: 49.2±7.6 ms, P<0.05 versus ZT4 and ZT21; and CaTD₈₀: ZT4: 70.1±3.3 ms; ZT9: 72.7±2.7 ms; ZT14: 64.3±3.3 ms; ZT21: 74.4±1.2 ms, P<0.05 versus other time points). The pacing frequency at which CaT alternans emerged was faster, and average CaT alternans magnitude was significantly reduced at ZT14 compared with the other time points. There was a trend for decreased spontaneous premature ventricular complexes and pacing-induced ventricular arrhythmias at ZT14, and the hearts at ZT14 had diminished responses to isoproterenol compared with ZT4 (ZT4: 49.5.0±5.6% versus ZT14: 22.7±9.5% decrease in APD, P<0.01). In contrast, aged hearts exhibited no difference between ZT14 and ZT4 in nearly every parameter assessed (except APD₈₀: ZT4: 39.7±1.9 ms versus ZT14: 33.8±3.1 ms, P<0.01). Gene expression of KCNA5 (potassium voltage-gated channel subfamily A member 5; encoding Kv1.5) was increased, whereas gene expression of ADRB1 (encoding β1-adrenergic receptors) was decreased at ZT14 versus ZT4 in adult hearts. No time-of-day changes in expression or phosphorylation of Ca²⁺ handling proteins (SERCA2 [sarco/endoplasmic reticulum Ca²⁺-ATPase], RyR2 [ryanodine receptor 2], and PLB [phospholamban]) was found in ex vivo perfused adult isolated hearts.

CONCLUSIONS

Isolated adult hearts have strong time-of-day variation in cardiac electrophysiology, Ca²⁺ handling, and adrenergic responsiveness, which is disrupted with age.

Aberrant sinus node firing during β-adrenergic stimulation leads to cardiac arrhythmias in diabetic mice

AIM

Cardiovascular complications, including cardiac arrhythmias, result in high morbidity and mortality in patients with type-2 diabetes mellitus (T2DM). Clinical and experimental data suggest electrophysiological impairment of the natural pacemaker of the diabetic heart. The present study examined sinoatrial node (SAN) arrhythmias in a mouse model of T2DM and physiologically probed their underlying cause.

METHODS

Electrocardiograms were obtained from conscious diabetic db/db and lean control db/+ mice. In vivo SAN function was probed through pharmacological autonomic modulation with isoprenaline, atropine and carbachol. Blood pressure stability and heart rate variability (HRV) were evaluated. Intrinsic SAN function was evaluated through ex vivo imaging of spontaneous Ca²⁺ transients in isolated SAN preparations.

RESULTS

While lean control mice showed constant RR intervals during isoprenaline challenge, the diabetic mice experienced SAN arrhythmias with large RR fluctuations in a dose-dependent manner. These arrhythmias were completely abolished by atropine pre-treatment, while carbachol pretreatment significantly increased SAN arrhythmia frequency in the diabetic mice. Blood pressure and HRV were comparable in db/db and db/+ mice, suggesting that neither augmented baroreceptor feedback nor autonomic neuropathy is a likely arrhythmia mechanism. Cycle length response to isoprenaline was comparable in isolated SAN preparations from db/db and db/+ mice; however, Ca²⁺ spark frequency was significantly increased in db/db mice compared to db/+ at baseline and after isoprenaline.

CONCLUSION

Our results demonstrate a dysfunction of cardiac pacemaking in an animal model of T2DM upon challenge with a β-adrenergic agonist. Ex vivo, higher Ca²⁺ spark frequency is present in diabetic mice, which may be directly linked to in vivo arrhythmias.

Uremia increases QRS duration after β-adrenergic stimulation in mice

Chronic kidney disease (CKD) and uremia increase the risk of heart disease and sudden cardiac death. Coronary artery disease can only partly account for this. The remaining mechanistic links between CKD and sudden death are elusive, but may involve cardiac arrhythmias. For the present study, we hypothesized that a thorough electrophysiological study in mice with CKD would provide us valuable information that could aid in the identification of additional underlying causes of sudden cardiac death in patients with kidney disease. Partial (5/6) nephrectomy (NX) in mice induced mild CKD: plasma urea in NX was 24 ± 1 mmol/L (n = 23) versus 12 ± 1 mmol/L (n = 22) in sham-operated control mice (P < 0.05). Echocardiography did not identify structural or mechanical remodeling in NX mice. Baseline ECG parameters were comparable in conscious NX and control mice; however, the normal 24-h diurnal rhythm in QRS duration was lost in NX mice. Moreover, β-adrenergic stimulation (isoprenaline, 200 μg/kg intraperitoneally) prolonged QRS duration in conscious NX mice (from 12 ± 1 to 15 ± 2 msec, P < 0.05), but not in sham-operated controls (from 13 ± 1 to 13 ± 2 msec, P > 0.05). No spontaneous arrhythmias were observed in conscious NX mice, and intracardiac pacing in anesthetized mice showed a comparable arrhythmia vulnerability in NX and sham-operated mice. Isoprenaline (2 mg/kg intraperitoneally) changed the duration of the QRS complex from 11.2 ± 0.4 to 11.9 ± 0.5 (P = 0.06) in NX mice and from 10.7 ± 0.6 to 10.6 ± 0.6 (P = 0.50) in sham-operated mice. Ex vivo measurements of cardiac ventricular conduction velocity were comparable in NX and sham mice. Transcriptional activity of Scn5a, Gja1 and several profibrotic genes was similar in NX and sham mice. We conclude that proper kidney function is necessary to maintain diurnal variation in QRS duration and that sympathetic regulation of the QRS duration is altered in kidney disease.

Advances in the relationship between Kruppel-like factor 15 and cardiovascular disease research

Kruppel-like factor 15 (KLF15) is a subtype of the Kruppel-like family of transcription factors (KLFs). KLFs have three high-fidelity zinc fingers at the carboxyl terminus that enable them to regulate the biological processes of proliferation, differentiation, cellular development, and apoptosis. KLF15 is highly expressed in the kidney, pancreas, and cardiac and skeletal muscle, and plays an essential role in the development and occurrence of multiple system diseases. In this paper, we underscored the important relationship between KLF15 and cardiovascular diseases such as atherosclerosis, heart failure, arrhythmia, aortic lesions, etc. On this basis, we identified KLF15 as a potential therapeutic target for the treatment of cardiovascular disease.