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

Prevalence and time course of arrhythmia-induced cardiomyopathy in patients with newly diagnosed heart failure and concomitant tachyarrhythmia – the TACHY-HF pilot trial

Background

Arrhythmias may often be a result of heart failure, but they can also cause left-ventricular systolic dysfunction (LVSD), thereby presenting as arrhythmia-induced cardiomyopathy (AIC). AIC-diagnosis is established retrospectively when LVSD normalizes or improves significantly over time following rhythm restoration. However, the prevalence and most importantly the time course of this relevant disease remain unclear and hence merit investigation to enable the correct diagnosis.

Purpose

Therefore, our aim was to evaluate a) the occurrence of AIC in this clinical relevant cohort of patients with newly diagnosed and otherwise unexplainable LVSD and concomitant tachycardia and b) the time needed to fulfill the diagnostic criteria of AIC in order to facilitate a diagnostic algorithm.

Method

We prospectively screened patients hospitalized for newly diagnosed and otherwise unexplainable LVSD (i.e. left ventricular ejection fraction (LVEF) <50%) and coexisting tachyarrhythmia (atrial fibrillation/flutter + heart rate (HR) >100/min) in 3 cardiological centers. Coronary angiography and cardiac magnetic resonance imaging were performed to exclude other causes for LVSD. Patients underwent a rhythm control strategy in accordance to the local clinical pathways. LVEF was assessed by echocardiography at presentation and at follow-up (FU) visits after 2, 4, and 6 months. Patients who lost sinus rhythm (SR) during FU were excluded. Patients with any increase of ≥15% in absolute EF or an EF ≥50% with an improvement of ≥10% after 6 months of FU were assigned to the AIC-group, which is a common definition of AIC. All others were assigned to an idiopathic DCM-group as final comparator.

Results

68 patients were eligible, 18 of them were excluded: 1 lost to follow-up, 1 PCI, 2 COVID-19, 1 diagnosed cancer, 1 withdraw consent and 12 lost SR. Thus, our sample consists of a total of 50 patients. At presentation, mean±SD HR was 121±17/min. After rhythm therapy, HR normalized (67±10/min) and LVEF increased in both groups, see fig. 1. Surprisingly, only 9 patients did not fulfill the AIC-criteria in this specific collective resulting in a prevalence of 82% (95%-CI: 68% – 92%). This high prevalence of AIC underlines the importance of the disease. 2 and 4 months after rhythm intervention, 58% and 73% of patients fulfilled AIC-criteria (fig. 2). The sensitivity for detection of AIC by echocardiographic LVEF-measurement at months 2 and 4 of FU was 65% and 86% with a specificity of 100%, emphasizing that a FU of 6 months is necessary to certainly distinguish between AIC and idiopathic DCM.

Conclusion

The prevalence of AIC in patients with newly diagnosed and otherwise unexplainable LVSD with concomitant tachycardia is 82%. Analysis of the time course of AIC clearly suggests that the final diagnosis cannot be established before 6 months after successful rhythm restoration. These results may help to improve diagnosis of AIC in daily clinical practice.

Funding Acknowledgement

Type of funding sources: None.

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

Genetic variants in calcium regulatory cardiac genes and their contribution to Takotsubo syndrome

Background and purpose

Takotsubo syndrome (TTS) is characterized by an acute left ventricular dysfunction similar to a myocardial infarction (MI) in the absence of coronary artery stenosis. Patients show symptoms similar to the acute MI with increased biomarkers and blood serum catecholamines. Recently, we developed a patient-specific TTS stem cell model and identified a higher sensitivity to catecholamine-induced stress. Furthermore, familial TTS cases and genetic studies point to a genetic predisposition. The purpose of this study was to analyze a genetic predisposition by characterizing genetic variants in genes associated with cardiac pathologies and their impact on calcium homoeostasis in TTS.

Methods and results

Whole exome sequencing analysis of a TTS patient discovered 2 missense AHNAK variants in its C-terminal domain and in addition the missense variant F189L in the calcium buffering calsequestrin 2 gene (CASQ2). AHNAK is a 700kDa big nucleoprotein and is involved in the β-adrenergic regulation of the cardiac calcium channel Cav1.2. 3-month old TTS-iPSC-derived cardiomyocytes (CM) were generated and the variants were confirmed by sequencing. We found AHNAK higher expressed in TTS-iPSC-CMs compared to control, whereas no expression alteration was observed for Cav1.2. Since AHNAK is described to act as a repressor towards Cav1.2, which is relieved under β-adrenergic stimulation, we analyzed the effect of AHNAK variants on a potential co-localization and interaction between both proteins. AHNAK and Cav1.2 were shown to co-localize in the cytoplasm as well as the membranes and co-immunoprecipitation experiments confirmed an interaction of AHNAK and Cav1.2 in all tested control- and TTS-iPSC-CMs. On a functional level, we were able to show by patch clamp analysis that Cav1.2 calcium currents are significantly increased in TTS-iPSC-CMs compared to control. The influence of CASQ2-F189L on sarcomeric reticulum (SR) calcium load was analyzed by epifluorescence microscopy using FURA4 and caffeine-applications. We found significantly decreased SR calcium content with an increased fractional release during systole in TTS-iPSC-CMs. To test, whether these variants are the main reason for altered interaction of AHNAK and Cav1.2, calcium currents or SR calcium load in TTS need to be proven in the future by using CRISPR/Cas9-rescued AHNAK/CASQ2 lines.

Conclusion

Here we show the cardiac functional consequences of AHNAK and CASQ2 missense mutations in TTS-iPSC-CMs with regard to calcium currents and SR calcium load. These results show that AHNAK and CASQ2 variants may predispose to TTS and enable a new therapeutic option for TTS.

Funding Acknowledgement

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

Dysfunctional crosstalk of cardiomyocytes and cardiac fibroblasts in a pluripotent stem cell model of dilated cardiomyopathy

Background/Purpose

Dilated cardiomyopathy (DCM) is characterized by left ventricular dilation and contractile dysfunction. Fibrosis is one major phenotypic result in DCM, pointing to the contribution of both, cardiomyocytes (CM) and cardiac fibroblasts (cFB) to DCM. The molecular basis of most DCM cases remains unknown. Nevertheless, it is known that up to 35% of all cases have a family history, linked to mutations in more than 30 gene loci. The aim of this study is to analyse the crosstalk of iPSC-CM and cFB and the underlying genetic and molecular causes in a patient-specific induced pluripotent stem cell (iPSC) model of DCM.

Methods and results

For this purpose a 4-member family was recruited containing 2 patients (father and daughter) with severe DCM and heart transplantation. iPSCs of all family members were generated and differentiated into iPSC-CMs. All iPSC-CMs express general cardiac markers, e.g. βMHC, α-actinin. Interestingly, αMHC expression was decreased in diseased iPSC-CMs in comparison to control cells. Additionally, the sarcomeric regularity was decreased in diseased iPSC-CMs. As we found significantly increased fibrosis (22%) in explanted myocardium of the diseased father compared to healthy myocardium (8%), both cFB and CM seem to play an important role. From the same myocardium primary cFBs were isolated and shown to express typical cFB markers clearly distinguishing these cells from non-fibroblasts as well as from fibroblasts with different origin. To analyse the contribution of cFBs and CMs to DCM on a functional level, 3D engineered heart muscles (EHMs) were generated in different diseased/healthy cell combinations. EHMs composed of both or either one affected DCM-iPSC-CMs and/or DCM-cFBs in comparison to healthy control EHMs did not produce any measurable force, indicating that the DCM-EHM phenotype is clearly diseased. Evaluation of tissues' viscoelasticity showed that DCM-cFB, DCM-iPSC-CMs and DCM-EHMs were stiffer than healthy control EHMs. Thus these data suggest that apart from the obvious dysfunction of DCM CMs, DCM cFBs clearly contribute to the contractile pathophysiology in DCM EHMs. Furthermore, whole exome sequencing of iPSCs was conducted to identify disease-causing variants. This analyses point towards a new genetic variant in the FLNc gene coding for a protein important in development, stabilization and maintenance of myofibrils. Rescue of this variant by CRISPR Cas9 genome editing will shed more light onto the role of this variant during DCM development in the future.

Conclusion

Using a ps-iPSC-CM model of a 4-member family with two severe DCM patients, we could demonstrate a clear contribution of both cell types, iPSC-CMs and cFB, to the contractile pathophysiology of DCM. We identified a potentially disease-causing new variant in the FLNc gene, which may contribute to the impaired functionality within the diseased iPSC-CMs and EHM. This makes FLNc a new therapeutic target for DCM.

Funding Acknowledgement

Type of funding source: Public grant(s) – National budget only. Main funding source(s): IRTG1816

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

SCN10A-knock-out improves survival and proarrhythmia in a transgenic heart failure mouse model

Background

In heart failure (HF) both Ca2+/Calmodulin-dependent protein-kinase II (CaMKII) and late sodium current (INaL) are known to contribute to arrhythmogenesis as they contribute to action-potential (AP) prolongation and the occurrence of early- (EADs) and delayed afterdepolarizations (DADs). Further, augmented CaMKII and INaL maintain a vicious cycle as they both can activate each other. We recently found that the sodium channel isoform NaV1.8 is upregulated in HF and hypertrophy and that it is involved in INaL-generation. In the current study we investigated the effects of NaV1.8-knock-out (KO) on HF-progression and arrhythmogenesis in a CaMKII-overexpressing HF mouse model.

Methods/Results

CaMKII overexpressing mice (CaMKII+/T) were crossbred with NaV1.8-KO mice (SCN10A−/−). To our surprise knock-out of NaV1.8 in CaMKII+/T mice (SCN10A−/−/CaMKII+/T) significantly improved survival (median survival 103 days vs 74.5 CaMKII+/T, p<0.01). CaMKII+/T mice exhibited a strong HF phenotype compared to WT with increased heart-weight to tibia length ratio as well as reduced ejection fraction and left-ventricular end-diastolic diameter obtained by echocardiography. However, these structural parameters did not differ between SCN10A−/−/CaMKII+/T and CaMKII+/T. Therefore, cellular electrophysiology experiments were performed in isolated cardiomyocytes for a better understanding of the observed improvement in survival. INaL, measured by patch-clamp technique, was significantly augmented in CaMKII+/T vs WT and SCN10A−/−, while SCN10A−/−/CaMKII+/T showed significantly less INaL than CaMKII+/T alone. Further, AP-duration (APD) was significantly reduced in SCN10A−/−/CaMKII+/T vs CaMKII+/T while AP-amplitude, resting membrane-potential and upstroke velocity (dv/dtmax) remained unchanged. In addition, the occurrence of afterdepolarizations was significantly lower in SCN10A−/−/ CaMKII+/T vs CaMKII+/T. Confocal microscopy using the dye Fluo-4AM was performed and significantly less diastolic Ca2+-waves occurred in SCN10A−/−/CaMKII+/T compared to CaMKII+/T. In order to analyze an organ-specific SCN10A-KO, we generated homozygous SCN10A-KO lines of induced pluripotent stem cells by using CRISPR/Cas9 technology. 2-month old iPSC-cardiomyocytes lacking NaV1.8 were treated with low dose isoprenaline and showed significantly less INaL, thereby serving as a final proof of the relevant role of this Na+-channel on INaL-generation in the cardiomyocyte.

Conclusion

We found a survival benefit by selective knock-out of the neuronal sodium channel isoform NaV1.8 in a proarrhythmic HF mouse model with augmented CaMKII expression. However, in our model NaV1.8-knock-out showed no effects on HF progression, while cellular proarrhythmic triggers were attenuated. Taken together with our findings in IPS-cardiomyocytes treated with the CRSIPR/Cas9 technology NaV1.8 plays a significant role for the generation of INaL and cellular arrhythmogenic triggers in the cardiomyocyte.

Funding Acknowledgement

Type of funding source: Foundation. Main funding source(s): Deutsche Stiftung für Herzforschung

P3829Antiarrhythmic effects of Sacubitrilat (LBQ657) on Ca2+ homeostasis in ventricular cardiomyocytes

Background and objectives

Simultaneous inhibition of neprilysin and angiotensin II receptors by sacubitril/valsartan was shown to significantly reduce morbidity and mortality in heart failure patients compared to sole interference with the renin angiotensin system. Beneficial effects of increased levels of natriuretic peptides following neprilysin inhibition have been suggested, whereas direct effects of sacubitrilat on myocardial Ca2+ cycling properties remain elusive.

Methods and results

Under basal conditions the combination of active neprilysin-inhibitior sacubitrilat (LBQ657) and angiotensin II receptor inhibitor valsartan did not influence diastolic Ca2+ spark frequency (CaSpF) nor arrhythmogenic SR Ca2+ leak in murine ventricular cardiomyocytes (confocal microscopy, n CMs/hearts=80/7 vs. 100/7, P=0.91/0.99). In contrast, sacubitrilat/valsartan treatment significanty reduced CaSpF by 35±9% and SR Ca2+ leak by 45±9% in CMs that had been put under catecholaminergic stress (isoproterenol 10nM, n=81/7 vs. 62/7, P<0.001 both). This effect could be clearly be attributed to the neprilysin inhibitor sacubitrilat as sole sacubitrilat treatment also reduced both parameters by similar degrees (reduction of CaSpF by 57±7% and SR Ca2+ leak by 76±5%; n=101/4 vs. 108/4, P<0.01 both) whereas sole valsartan treatment did not affect diastolic SR Ca2+ leak. Of note, systolic Ca2+ release, SR Ca2+ load and Ca2+ transient kinetics of murine CMs were not compromised upon treatment with sacubitrilat (epifluorescence microscopy, n=41/6 vs. 39/6). Importantly, sacubitrilat/valsartan in combination as well as sacubitrilat alone also reduced diastolic CaSpF and SR Ca2+ leak by 40–74% in human left-ventricular CMs from patients with end-stage heart failure (n=71/8 vs. 78/8, P<0.05).

Conclusion

This study demonstrates that neprilysin-inhibition directly exerts beneficial effects on Ca2+ homeostasis in human heart failure. We can show for the first time that neprilysin-inhibition by sacubitrilat yields a strong reduction of arrhythmogenic SR Ca2+ leak without affecting systolic Ca2+ release. These effects might contribute to the mortality benefit of sacubitril/valsartan treatment in the PARADIGM Study.

Acknowledgement/Funding

THF was funded by the Deutsche Forschungsgemeinschaft (DFG) through the SFB 1002 (A11). SS is supported by the Marga und Walter Boll-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.

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).

P1230Impact of regulated junctophilin-2 clustering at axial tubule junctions on atrial excitation-contraction coupling and therapeutic implications

Objectives

Atrial dysfunction is highly prevalent and known to significantly aggravate heart failure. While rapid excitation-contraction (EC) coupling depends on axial tubule junctions in atrial myocytes (AMs), the mechanisms leading to atrial loss-of-function remain unclear. Junctophilin-2 (JP2), a tail-anchored protein of the sarcoplasmic reticulum, stabilizes the integrity of ventricular Ca2+ release units, which is disrupted in ventricular myocytes by reduced JP2 expression or proteolysis. Here we aim to characterize the abundance and subcellular localisation of JP2 in AMs, to assess the impact of decreased JP2 expression on atrial remodelling, and to investigate the potential to correct JP2 expression and atrial dysfunction.

Results

We identified 5-fold lower JP2 levels in atrial compared to ventricular tissue in mouse and human hearts by SDS-PAGE. Surprisingly, in AMs, this resulted in subcellular expression of large JP2 clusters at axial tubule junctions together with highly phosphorylated ryanodine receptor (RyR2) channels visualized by STED superresolution microscopy. Importantly, left atrial hypertrophy induced by aortic pressure overload led to an additional strong decrease in JP2 expression compared to sham control, disrupted junctional RyR2 clustering and EC-coupling. This loss-of-function mechanism was confirmed by conditional shRNA-mediated JP2 knockdown. Quantitative image analysis after atrial JP2 knockdown showed a 50% decrease in area overlap between RyR2 and JP2 in AMs (JP2 knockdown 0.03±0.003 μm2 vs. control 0.06±0.004 μm2, p<0.001), and a ∼2-fold increased Ca2+ spark frequency, consistent with decreased left atrial fractional shortening (JP2 knockdown 12.9±0.8% vs. control 16.5±0.9%, p<0.01). Whereas atrial-ventricular dysfunction due to aortic pressure overload resulted in 40% mortality, additional JP2 knockdown exacerbated mortality to 100% (n: 10 control vs. 9 JP2 knockdown mice). In contrast, transgenic JP2 overexpressor mice showed greatly improved atrial contractility without mortality after induced aortic pressure overload (n: 21 control vs. 16 JP2 overexpressor mice). JP2-OE not only augmented atrial RyR2-clustering, but induced the de-novo biogenesis of large poly-adic junctional membrane complexes, which were resolved by STED microscopy via high-resolution cholesterol-based membrane staining in live AMs and electron tomography.

Conclusions

Nanoscale imaging identifies a new subcellular mechanism of significantly limited atrial JP2 protein expression in large clusters at axial tubule junctions. In atrial hypertrophy, JP2 is further decreased with junctional RyR2 cluster disruption leading to impaired Ca2+ release and decreased contractility. Importantly, JP2 overexpression effectively protected from atrial dysfunction, providing a novel therapeutic rationale for atrial cardiomyopathies.

4966Targeting INaL by a neuronal sodium channel isoform improves survival in a CaMKII-transgenic heart failure mouse model

Background

Cardiac pathologies like hypertrophy and heart failure are known to be associated with proarrhythmogenic triggers like early- (EADs) and delayed afterdepolarizations (DADs) that can be partly attributed to an augmentation of late sodium current (INaL). Enhanced INaL is closely connected with increased activity of Ca2+/calmodulin dependent-kinase II (CaMKII) in pathology as it is enhanced by CaMKII on the one hand but can also indirectly increase CaMKII-activity on the other. We recently found neuronal sodium channel NaV1.8 to be involved in INaL-augmentation in heart failure and cardiac hypertrophy. Here, we studied possible antiarrhythmic effects of NaV1.8-inhibition in a transgenic mouse model with enhanced CaMKII-expression by selectively knocking out NaV1.8.

Methods/Results

To investigate antiarrhythmic effects of NaV1.8-depletion in-vivo and in-vitro we crossbred CaMKII-transgenic mice (CaMKII+/T) with NaV1.8-knock-out mice (SCN10A−/−). Surprisingly, CaMKII+/T-mice lacking NaV1.8 (CaMKII+/T & SCN10A−/−) showed a significantly improved survival compared to CaMKII+/T alone (97.5 vs 72.0 days, p<0.05). Heart weight to tibia length ratio was significantly increased in CaMKII+/T-mice compared to wild-type, without any differences between CaMKII+/T and CaMKII+/T & SCN10A−/−. To investigate the underlying mechanisms out of this observation we isolated single cardiomyocytes and performed patch-clamp experiments as well as confocal microscopy to measure Ca2+-transients and diastolic Ca2+-waves. INaL-integral was significantly smaller in cardiomyocytes from CaMKII+/T & SCN10A−/−-mice compared to CaMKII+/T alone. During action potential recordings, significantly less afterdepolarizations occurred in CaMKII+/T & SCN10A−/− compared to cardiomyocytes from CaMKII+/T -mice (16.7/min vs 34.9/min, p<0.05). There was a trend of less cells exhibiting diastolic Ca2+-waves in Ca2+-measurements from CaMKII+/T & SCN10A−/− compared to CaMKII+/T (15% vs 25%, p=0.09). As some cells showed more than one event, we calculated the frequency of Ca2+-waves and found a significant reduction of Ca2+-waves in CaMKII+/T & SCN10A−/− vs. CaMKII+/T (22.8/min vs 43.0/min, p<0.05). Moreover, the time to the first event was significantly longer in CaMKII+/T & SCN10A−/−. Ca2+-transient amplitude (F/F0) was significantly lower in CaMKII+/T compared to CaMKII+/T & SCN10A−/− (4.6 vs. 5.3, p=0.05). Further, Ca2+-extrusion from the cytosol was significantly faster in CaMKII+/T & SCN10A−/−.

Conclusion

Our data demonstrates, that inhibition of INaL by targeting NaV1.8 has a potent antiarrhythmic potential as we found a reduction of EADs, DADs and diastolic Ca2+-waves in CaMKII+/T & SCN10A−/−-cardiomyocytes. This antiarrhythmic potential appears to be potent enough to improve survival and to rescue the proarrhythmogenic phenotype of CaMKII-overexpression. However, further in-vivo experiments are necessary to investigate NaV1.8-inhibition for a possible therapeutic approach.

Effects of different proton pump inhibitors on cardiac contractility in isolated human failing myocardium

AIM

Proton pump inhibitors (PPI), e.g. pantoprazole (PP), esomeprazole (EP) and omeprazole (OP), work as anti-ulcer/gastrointestinal reflux drugs. Also, they are widely used in postoperative care of patients in cardiac surgery to prevent upper gastrointestinal bleeding. Therefore, in western industrial countries they play a major economic role, representing one of the most important drugs in open heart cardiac surgery.

METHODS

Intact muscle strips (n=32) were isolated from the right ventricle wall of failing human hearts. In four different groups (PP, EP, OP, control group, each n=8), force amplitudes were recorded at a frequency of 60 beats per minute (bpm) with increasing PPI concentrations (0 to 320 µm/mL).

RESULTS

In isometrically contracting muscle strips, significant negative inotropic effects were observed in the presence of all three PPI-groups (PP, EP and OP) with doses of 2.5 µg/mL and higher compared to the control group (p < 0.05 each). With high doses (320 µm/mL), force amplitudes could be almost completely depressed. The half maximal inhibitory concentration (IC50) for EP was 35.7 (confidence interval: 17.3-73.6) vs. OP 29.3 (6.8-126.6) vs. PP 25.1 (14.6-43.1) µg/mL (n.s.). No significant differences were found between the different proton pump inhibitors (PP, EP, OP) throughout the range of all concentrations. Relaxation was impaired in all PPI subgroups with prolonged time to 90% relaxation (RT90%) and maximum relaxation velocity (‑df/dt) was reduced, too. These effects were partially reversible after wash-out of the drugs.

CONCLUSION

We conclude that proton pump inhibitors show significant negative inotropic effects on isolated human failing myocardium. There is no apparent difference seen in the magnitude of the effects of each PPI-group. Further, in-vivo investigations are necessary to reveal the clinical evidence of PPI's negative inotropic effects, e.g. in cardio-surgical patients with heart failure.