Electrophysiological remodeling in tachycardia-induced cardiomyopathy

Background

Tachycardia-induced cardiomyopathy (TCM) is a reversible and likely underrecognized form of heart failure. Thus, a better understanding of the TCM-pathophysiology is warranted as the underlying early mechanisms that mediate the progression of TCM remain unclear.

Purpose

This study aimed to identify the cellular mechanisms of TCM.

Methods and results

Human induced pluripotent stem cell cardiomyocytes (iPSC-CM) were utilized as a translational human-based model. We performed chronic tachycardic (120 bpm) or normofrequent (control, 60bpm) cell culture pacing to study cellular changes during TCM progression.

Already after 24h of tachycardic stimulation of iPSC-CM, we detected a decrease in Ca transient amplitude compared to control (Fura-2, n=49/44 cells/9 differentiations). Diastolic Ca levels and cytosolic Ca elimination were not affected after 24h of tachycardia (n=49/44/9). We detected no difference in sarcoplasmic reticulum (SR) Ca load (assessed via caffeine application) or SERCA activity (Ksys-Kcaff) after 24h of tachycardia (n=13/15/5). However, demonstrating the progress of TCM, 7d of tachycardia resulted in progressive decline of Ca transient amplitude together with an impaired Ca elimination, while diastolic Ca concentration was unchanged (n=73/66/8). These changes may underlie the reduced systolic force and impaired relaxation in TCM. We could explain these results by a significantly reduced SR Ca load and a diminished SERCA activity after 7d tachycardia (n=13/7 vs. 13/4). Using confocal microscopy (Fluo-4) we detected no difference in SR Ca spark frequency after 24h of tachycardia (n=82/66/8), while 7d of tachycardia caused an increase of Ca spark frequency (n=76/79/7), which is a typical hallmark of maladaptive remodeling in HF and likely underlie the reduced SR Ca load. Voltage clamp data of late Na current (INaL) showed no difference in INaL after 24h of stimulation (n=17/6 vs. 19/7), whereas INaL was increased after 7d of tachycardia (n=26/7 vs. 19/6). Accordingly, whole-cell current clamp experiments revealed a prolongation of the action potential after 7d of tachycardia compared to control (n=21/6 vs. 19/5), while no difference of action potential duration could be detected after 24h (n=37/31/8). Resting membrane potential and action potential amplitude were not changed. Finally, we investigated tachycardia-mediated effects on explanted human failing hearts. 8h of tachycardic stimulation (120 bpm) of human failing ventricular trabeculae already compromised systolic force, and diastolic tension and relaxation time were markedly increased compared to control (60bpm, n=8/6 trabeculae /7/6 human hearts).

Conclusion

This study demonstrates that persistent tachycardia adversely alters cardiomyocyte excitation-contraction coupling via electrophysiological cellular remodeling. Our translational investigation in human myocardium may help to understand the pathophysiology of an underrated but prevalent disease.

Funding Acknowledgement

Type of funding sources: Foundation. Main funding source(s): Else Kröner-Fresenius-Stiftung (EKFS)Deutsche Gesellschaft für Innere Medizin

Effects of atrial fibrillation on ventricular remodeling in the human heart

 

Atrial fibrillation (AF) is often found in patients with heart failure (HF). Clinical data indicated that the arrhythmic component of AF alone could contribute to left-ventricular (LV) dysfunction. However, the effects of non-tachycardic AF with arrhythmic excitation of the human LV, are unknown.

We investigated human LV myocardium from patients with sinus rhythm (SR) or normofrequent AF (mean EF>50%, matched clinical data, derived from septal resections during AVR). In histological analysis we detected no difference between SR (n=17 patients) and AF patients (n=18) regarding the amount and distribution of fibrosis. We isolated human LV cardiomyocytes (CM) and studied cellular Ca-handling (Fura-2). Systolic Ca-transient amplitude of LV CM was reduced in patients suffering from AF (n=8 AF patients vs. 11 SR), while diastolic Ca-levels and Ca-transient kinetics were not significantly changed. These results were confirmed in LV CM from non-failing donors (NF) with AF (n=4 AF patients vs. 8 SR). For the standardized investigation of a normofrequent arrhythmia, we simulated AF in vitro by using arrhythmic (60 bpm, 40% beat-to-beat variability) or rhythmic (60 bpm) field stimulation. Human LV CM from NF SR patients (n=8) showed an impaired Ca-transient amplitude after 24h arrhythmic culture pacing without changes in diastolic Ca and Ca-transient kinetics. For studying a model suitable for more standardized chronic pacing, we utilized human iPSC cardiomyocytes (iPSC-CM) from healthy donors (n=6). After 7 days, arrhythmically paced iPSC-CM exhibited a reduced systolic Ca-transient amplitude, a trend towards a prolonged Ca-elimination time and a reduced sarcoplasmic reticulum Ca-load. Confocal line-scans of arrhythmically paced cells (Fluo-4 AM) showed an increased diastolic Ca-leak from the sarcoplasmic reticulum, possibly underlying the reduced Ca-load. Coupled with the Ca changes, cytosolic Na was elevated after arrhythmia. We found an increased late INa, which could explain the detrimentally altered Ca/Na-interplay. Accordingly, Patch-clamp experiments revealed a prolonged action potential duration after arrhythmia. We further elucidated the underlying mechanisms of this electrophysiological remodeling by showing that oxidative stress (H2O2, LPO) is increased in the LV of patients suffering from AF (n=6 AF patients vs. 6 SR), which was associated with an enhanced NOX2/-4 activity. Consecutively, Ca2+/calmodulin-dependent protein kinase IIδ (CaMKII) was found to be more oxidized (CaMKII-Met281/282) in the LV of AF patients (n=7 AF patients vs. 7 SR) leading to an increased CaMKII activity, which adversely regulated EC-coupling protein phosphorylation including RyR2 hyperphosphorylation.

Normofrequent arrhythmia/AF impairs human ventricular EC-coupling via increased oxidative stress and enhanced CaMKII. Thus, this translational study provides the first mechanistic characterization and the potential negative impact of isolated AF on the human LV.

Funding Acknowledgement

Type of funding sources: Public Institution(s). Main funding source(s): Else Kröner-Fresenius-Stiftung (EKFS) and Deutsche Gesellschaft für Innere Medizin

Atrial fibrillation impairs ventricular function by altering excitation-contraction coupling in the human heart

 

Atrial fibrillation (AF) often co-exists in patients with heart failure (HF). Recent clinical evidence suggests that the arrhythmic component of AF alone may contribute to ventricular dysfunction. However, the pathophysiological effects of a non-tachycardic AF on the human ventricle are unknown. To investigate the effects of normofrequent AF on the human ventricle we investigated ventricular myocardium from patients with preserved ejection fraction with sinus rhythm (SR) or AF in the absence of HF (compensated hypertrophy, EF>50%, matched clinical characteristics). In histological analysis we detected no difference between SR (n=9) vs. AF (n=6) regarding the amount and distribution of fibrosis. For functional investigation, Ca-handling was studied (Fura-2 AM). While systolic Ca-transient amplitude was in trend reduced in isolated human ventricular AF cardiomyocytes, we found a significantly prolonged Ca-elimination time (n=17–22 cells/4 pat.). Using caffeine application, a decreased SR Ca-load in AF was detected, which may be explained by a significant decrease in SERCA2a activity (ksys-kCaff, n=10–12/4 pat.). Patch-clamp experiments revealed a prolonged action potential duration in AF cardiomyocytes (n=5/15 cells).

For the standardized evaluation of the mechanisms of persistent normofrequent arrhythmia, we simulated AF in vitro by using arrhythmic (1 Hz, 40% R-R-variability) or rhythmic (1 Hz) field stimulation. We performed contractility experiments using in-toto isolated human ventricular trabeculae from explanted human hearts. After 8h of pacing, arrhythmically stimulated human trabeculae showed a significantly reduced systolic force, an increase in diastolic tension and a prolonged relaxation (n=11–12 trabeculae/11 pat.). For studying the cellular effects of persistent normofrequent arrhythmia in a model suitable for chronic pacing (up to 7 days), we utilized human iPSC cardiomyocytes (iPSC-CM) from healthy donors (n=6). After 7 days, arrhythmic paced iPSC-CM showed a significantly reduced systolic Ca-transient amplitude, a prolonged Ca-elimination time (n=35/45 cells) as well as a reduced SR Ca-load and a trend towards a lower SERCA2a activity compared to control (n=11 cells). Confocal line-scans (Fluo-4 AM) showed an increased diastolic SR Ca-release, which might also explain the reduced SR Ca-content (n=45/35 cells). Moreover, in irregularly paced iPSC-CM we found significant increased levels of cytosolic Na (n=69 cells each) and in patch-clamp experiments a significantly prolonged action potential duration (n=14/11 cells/3 diff.).

This study demonstrates that a normofrequent arrhythmic ventricular excitation as it occurs in AF impairs human ventricular myocardial function by altering cardiomyocyte excitation-contraction coupling. Thus, this study provides the first translational mechanistic characterization and the potential negative impact of isolated AF in the absence of tachycardia on the human ventricle.

Funding Acknowledgement

Type of funding source: None

CaMKII delta interaction with neuronal sodium channel Nav1.8 contributes to arrhythmogenic triggers in failing human and mouse cardiomyocytes

 

In heart failure, enhanced persistent current through neuronal sodium channel NaV1.8 (INaL) may induce influx of Na+ into cardiomyocytes. This may cause Ca2+ influx via the Na+/Ca2+ exchanger leading to increased proarrhythmogenic diastolic sarcoplasmic reticulum (SR) Ca2+ leak. This Ca2+ may activate Ca2+/calmodulin-dependent protein kinase IIδ (CaMKIIδ) which can induce INaL augmentation by phosphorylating NaV1.5 channels leading to a vicious cycle between INaL and CaMKIIδ.

Here, we examined whether CaMKIIδ associates with NaV1.8 in human and mouse cardiomyocytes thereby regulating its function. Interaction and co-localisation of CaMKIIδ and NaV1.8 were confirmed by co-immunoprecipitation and immunocytochemistry. Whole-cell patch clamp showed a potent reduction of INaL after addition of novel specific Nav1.8 blockers, either A-803467 (30 nmol/L) or PF-01247324 (1 μmol/L) in failing mouse cardiomyocytes overexpressing CaMKIIδc (CaMKIIδc+/T: −109.4±10.6 vs A-803467: −56.9±11.7 and PF-01247324:−-69.9±8.6 A*ms*F-1). In failing human cardiomyocytes inhibition of either NaV1.8 or CaMKIIδ using AIP (1 μmol/L) or AIP and PF-01247324 together led to a significant and comparable decrease of INaL (control: −93.7±7.1 vs PF-01247324: −56.8±6.6; AIP: −44.2±6.6; AIP+PF-01247324: −39.8±5.4 A*ms*F-1). Furthermore, to confirm whether observed alterations in INaL after inhibition of NaV1.8 are not due to an overall reduction in peak sodium current (INa) we measured INa properties in mouse cardiomyocytes. Importantly, we observed no difference neither in the peak nor in inactivation between wild type (WT), WT with PF-01247324 and in mice lacking NaV1.8. Using confocal microscopy we investigated whether inhibition of the NaV1.8-mediated INaL could attenuate the increase of proarrhythmogenic SR Ca2+ spark frequency (CaSpF) caused by overexpression of CaMKIIδ in mice. We observed a significant reduction of CaSpF in both NaV1.8 inhibitor groups (PF-01247324: 0.51±0.08 and A-803467: 0.57±0.08 μm–1 s–1) compared to control (1.00±0.13 μm–1 s–1). Incubation of human failing cardiomyocytes with either AIP (0.35±0.06 μm–1 s–1) or PF-01247324 (0.44±0.11 μm–1 s–1), or blocking CaMKIIδ and NaV1.8 together (0.30±0.08 μm–1 s–1) resulted in significant decrease of CaSpF compared to control (0.89±0.13 μm–1 s–1).

In conclusion, we show for the first time subcellular localisation of the neuronal sodium channel NaV1.8 and its interaction with CaMKIIδ in both human and mouse ventricular cardiomyocytes. Moreover, pharmacological inhibition of NaV1.8 caused a reduction of the augmented INaL and spontaneous diastolic SR-Ca2+ release in both failing human and mouse cardiomyocytes. NaV1.8 and CaMKIIδ interaction seem to play a relevant role for the generation of arrhythmogenic triggers (INaL & spontaneous diastolic SR-Ca2+ release) in both human and mouse cardiomyocytes from failing hearts.

Funding Acknowledgement

Type of funding source: None

Cellular mechanisms of early tachycardia-induced ventricular dysfunction in the human heart

Background

Tachycardia-induced cardiomyopathy (TCM) is a reversible form of ventricular dysfunction caused by persistent tachycardia. Characterization of TCM is mainly based on artificially RV paced animal models. Moreover, the underlying mechanisms and time course from compensation to failure remain unclear. This study aimed to investigate early cellular remodeling of tachycardia-induced myocardial dysfunction in human myocardium.

Methods and results

To elucidate early cellular electrophysiological targets mediating the transition to TCM, we chronically paced (120bpm vs 60bpm control) human induced pluripotent stem cell cardiomyocytes (hiPS-CM) for up to 7d. As a major substrate of cellular myocardial dysfunction, we investigated the influence of chronic tachycardia on cellular Ca cycling. After 24h of persistent tachycardia we detected a significant decrease in Ca transient (CaT) amplitude and reduced diastolic Ca levels (Fura-2). Meanwhile, Ca elimination time (RT80) was unchanged compared to control (n=44/42 cells / 8 diff.). Caffeine application was performed to evaluate sarcoplasmic reticulum (SR) Ca load. We found a shortening of caffeine-induced CaT relaxation time, whereas SR Ca load was unchanged (n=12/13 /8). Further illustrating the transition to TCM, CaT amplitude was progressively decreased after 7d of chronic tachycardia. In contrast to 24h of tachycardia, 7d persistent stimulation resulted in slowed relaxation (RT80, n=75/65 /7). These findings could be explained by a significant reduction of SERCA activity (Ksys-Kcaff) and SR Ca load (n=14/12 / 7). Diastolic Ca concentration remained reduced (n=75/65 /7), in total suggesting a shift to transsarcolemmal Ca elimination.

Sodium measurements (SBFI) revealed a significant increase of intracellular sodium concentration (n=69/69 /5) after 7d of tachycardia.

In patch clamp experiments we detected a prolongation of action potential duration as early as 24h after onset of tachycardia (n=26/21 /4), which persisted throughout 7d of pacing (n=8/12 /3). Resting membrane potential and action potential amplitude were not changed.

Finally, we investigated tachycardia-mediated effects on pre-existing human heart failure (HF). 8h tachycardic stimulation (120bpm) of human HF ventricular trabeculae compromised systolic force, while diastolic tension and relaxation time were markedly increased compared to control (60bpm) (n=7/6 trabeculae /6 human hearts).

The extensive molecular characterization of involved ion channels and pathways mediating transition to TCM is currently under investigation.

Conclusion

This study demonstrates that a persistent tachycardia adversely alters cardiomyocyte excitation-contraction coupling via early electrophysiological cellular remodeling. In pre-existing HF persistent tachycardia strongly aggravates ventricular dysfunction. Our first translational investigation in human myocardium may help to understand the pathophysiology of an underrated and very prevalent disease.

Funding Acknowledgement

Type of funding source: Foundation. Main funding source(s): Else-Kröner-Fresenius-Stiftung

4967Effects of atrial fibrillation on the human ventricle

Background

The consequence of normofrequent atrial fibrillation (AF) on the ventricle remains largely unknown.

Methods and results

To elucidate the effects of arrhythmic excitation on human ventricular myocardium we performed contractility experiments using ventricular trabecula from patients with heart failure (HF). Normofrequent AF was simulated using arrhythmic (60 bpm, 40% R-R interval variability) or rhythmic field stimulation (60 bpm). Within 8h of arrhythmic stimulation, human specimen showed an impaired systolic force, while diastolic tension increased pathologically (n=5–7 each/7 HF patients, Fig. 1). The characterization of the ventricular (in-vivo) phenotype in patients with AF was performed by utilizing ventricular myocardium from patients with sinus rhythm (SR) and from patients with AF in the absence of HF (compensated hypertrophy, EF>50%, matched clinical characteristics, LV myocardium obtained from aortic valve replacement surgery). Histological investigation showed increased levels of interstitial fibrosis in myocardium from patients with AF compared to SR (n=10 patients each). Studies of cellular Ca-homeostasis (epifluorescence microscopy, Fura-2) were performed using isolated human ventricular cardiomyocytes. While systolic Ca-transient amplitude (0.5 Hz) was preserved in ventricular cardiomyocytes from patients with AF, we found a significantly prolonged Ca-elimination time (RT80) by 22.0±7.7% and a trend towards increased diastolic Ca-levels (n=17–23 cells/4 patients each). This finding may be explained by a decrease in SERCA2a activity (ksys-kCaff, n=10–12/4 each) and an enhanced phospholamban expression in Western Blot experiments (n=5 patients each). For the standardized investigation of the involved targets/mechanisms mediating the pathological changes upon arrhythmic excitation, we utilized human induced pluripotent stem cell cardiomyocytes (iPSC-CM) from healthy donors for chronic arrhythmic culture stimulation (24h). Arrhythmic paced iPSC-CM showed no changes in systolic Ca-transient amplitude (0.5 Hz), whereas diastolic Ca-levels were increased, which fits nicely to the finding of disturbed trabeculae diastolic function (n=15 cells each). In patch clamp experiments, arrhythmic paced cells showed no alterations of resting membrane potential, upstroke velocity, action-potential amplitude or -duration (n=7–9 cells each). Protein expression levels of key Ca-handling proteins in iPSC-CM as well as regulated genes are already under investigation.

Conclusion

This study demonstrates that arrhythmic ventricular excitation deteriorates human myocardial contractility early in HF. In biopsies from patients with preserved EF, chronic AF was associated with increased levels of interstitial fibrosis and pathological diastolic Ca-handling, which could be causally confirmed in chronically arrhythmic paced iPSC-CM. Therefore, this study provides first mechanistic characterization of AF mediated effects on the human ventricle.

P1596Interaction of CaMKII and NaV1.8 modulates cardiac electrophysiology in human heart failure

Introduction

In human heart failure, electrical remodeling contributes to the risk of arrhythmia generation. Increased expression of Ca/Calmodulin-dependent protein kinase IIδ (CaMKIIδ) and an enhanced persistent Na current (INaL) have been linked to arrhythmogenesis. CaMKIIδ increases INaL via regulation of sodium channels thereby contributing to arrhythmias through early- and delayed-afterdepolarizations (EADs and DADs). Genome-wide association studies (GWAS) have described the implication of the neuronal sodium channel isoform NaV1.8 (SCN10A) in cardiac electrophysiology showing modulation in cardiac conduction. We showed that the expression of the isoform Nav1.8 is significantly increased in human failing cardiomyocytes and contributes substantially to the enhanced INaL.

Purpose

We investigated a potential interaction of CaMKIIδ and NaV1.8 and thereby its role in arrhythmia generation and electrophysiology in human and murine failing hearts.

Methods

Cardiomyocytes were isolated from explanted failing hearts and CaMKIIδ transgenic (TG) mice. We performed immunostainings and co-immunoprecipitation (Co-IP) to show interactions of CaMKIIδ and Nav1.8 in isolated cardiomyocytes and homogenates. Whole-cell patch clamp experiments were conducted in isolated human and murine ventricular cardiomyocytes. Additionally, Ca2+ transients were measured using epifluorescence microscopy with the Ca2+ dye fura-2 (10μmol/L) whereas Ca2+ sparks measurements were performed by using confocal microscopy with the Ca2+ dye fluo-4 (10μmol/L). PF-01247324 is a novel specific NaV1.8 inhibitor (orally bioavailable; 1 μmol/L) and autocamtide inhibitory peptide (AIP, 1 μmol/L) was used to inhibit CaMKIIδ.

Results

Co-immunoprecipitation experiments revealed an association of CaMKIIδ and Nav1.8 in human homogenates compared to healthy controls. Furthermore, immunohistochemistry stainings in isolated human cardiomyocytes showed a co-localization of CaMKIIδ and NaV1.8 at the intercalated disc and t-tubules. We observed a significant reduction of INaL integral and proarrhythmic SR-Ca2+ spark frequency (CaSpF) after addition of either PF-01247324 or the CaMKIIδ inhibitor AIP in failing human and murine ventricular cardiomyocytes. When PF-01247324 and AIP were added together, the decrease in INaL integral and CaSpF was comparable to PF-01247324 alone in human failing cardiomyocytes. Inhibition of NaV1.8 did not show an effect on Ca2+ transient amplitude or Ca2+ transient decay at different stimulation frequencies in CaMKIIδ TG cardiomyocytes.

Conclusion

Our results demonstrate the significance of both CaMKIIδ and NaV1.8 in INaL generation and their detrimental interaction. This data suggest that increased CaMKIIδ activity plays a substantial role for the activation of NaV1.8-mediated late sodium current and SR-Ca2+ leak.

P3832Dantrolene reduces CaMKII-mediated arrhythmogenesis

Rationale

In atrial and ventricular rhythm disorders, an increased diastolic sarcoplasmatic reticulum (SR) calcium leak can induce a depolarizing transient inward current, serving as a trigger for cellular arrhythmias. Dantrolene has been shown to also stabilize the cardiac ryanodine receptor. However, the detailed mechanism of the mode of action remains unknown. This study aims to investigate the effects of dantrolene on calcium/calmodulin-dependent kinase II (CaMKII) mediated arrhythmogenesis.

Methods and results

Right atrial cardiomyocytes (CM) were isolated from patients with atrial fibrillation. To investigate SR Ca2+ leak, measurements of diastolic SR Ca2+ sparks were performed by confocal microscopy using Fluo-4 AM. Dantrolene (10 μmol/l) potently reduced Ca2+-spark-frequency (CaSpF) by 90±26% (p<0.05, n=21 cells dantrolene vs. 19 cells control) leading to a reduction of the calculated diastolic SR-Ca2+-leak by 91±31% (p<0.05, n=21 vs. 19). Interestingly, CaMKII-inhibition using Autocamtide-2-Related Inhibitory Peptide (AIP) did not further reduced SR Ca2+ leak compared to dantrolene alone in human cardiomyocytes. This observation may suggest (secondary) inhibitory effects of dantrolene on CaMKII. To elucidate the role of CaMKII in dantrolene-mediated antiarrhythmic effects, we investigated atrial CM from mice overexpressing CaMKII (TG) and respective wildtype controls (WT). CaSpF and SR Ca2+ leak were reduced by dantrolene in both TG and WT mice (p<0.005, TG: dantrolene vs. vehicle n=132 vs 127 cells (9 mice); WT: dantrolene vs. vehicle n=61 vs 61 cells (5 mice)). However, proarrhythmogenic Ca2+ waves were only significantly reduced by dantrolene in TG mice (p<0.05, TG: dantrolene vs. vehicle 10.8% vs. 26.2%, n=154 vs 164 cells). Correspondingly, the incidence of delayed afterdepolarizations (DADs) in TG cells was significantly diminished by dantrolene (p<0.05, TG: dantrolene vs. vehicle 1/14 vs. 9/15 cells, n=5 mice). In contrast, DADs were not reduced by dantrolene in WT cells without increased CaMKII activity (p=n.s., WT: dantrolene vs vehicle 3/16 vs 2/13 cells, n=5 mice). In preliminary in vivo experiments, intraperitoneal injection of 40 mg/kg body weight dantrolene reduced the inducibility of arrhythmias by ventricular burst stimulation in CaMKII TG mice compared to vehicle (dantrolene 0/2 mice vs. vehicle 2/2 mice, p<0.05 Chi-Square).

Conclusion

Dantrolene beneficially altered Ca2+ homeostasis in human AF CM and murine CM. Dantrolene seems to exert its antiarrhythmic potential in a CaMKII-dependent manner. Thus, dantrolene as an already clinically approved compound might be a potential antiarrhythmic drug that merits clinical investigation.

Acknowledgement/Funding

Deutsche Forschungsgemeinschaft (MA 1981/5-1 and 7-1 to LSM). Marga und Walter Boll-Stiftung (SS).