Hypercontractile cardiac phenotype in mice overexpressing the regulatory subunit PR72 of protein phosphatase 2A

Background

The activity, localization, and substrate specificity of the protein phosphatase 2A (PP2A) heterotrimer are controlled by various regulatory B subunits. PR72 belongs to the B'' gene family and has been shown to be upregulated in human heart failure. However, little is known about the functions of PR72 in the myocardium.

Methods

To address this issue, we generated a transgenic mouse model with heart-specific overexpression of PP2A-PR72. Biochemical and physiological methods were used to determine contractility, Ca2+ cycling parameters, and protein phosphorylation.

Results

A 2.5-fold increase in PR72 expression resulted in moderate cardiac hypertrophy. Maximal ventricular pressure was increased in catheterized transgenic mice (TG) compared to wild-type (WT) littermates. This was accompanied by an increased shortening of sarcomere length and faster relaxation at the single-cell level in TG. In parallel with these findings, the peak amplitude of Ca2+ transients was increased, and the decay in intracellular Ca2+ levels was shortened in TG compared to WT. The changes in Ca2+ cycling in TG were also evident from an increase in the full duration and width at half maximum of Ca2+ sparks. Consistent with the contractile data, phosphorylation of phospholamban at threonine-17 was higher in TG hearts. The lower expression of the Na+/Ca2+ exchanger may also contribute to the hypercontractile state in transgenic myocardium.

Conclusion

Our results suggest that PP2A-PR72 plays an important role in regulating cardiac contractile function and Ca2+ cycling, indicating that the upregulation of PR72 in heart failure is an attempt to compensate functionally.

Impaired myocellular Ca2+ cycling in protein phosphatase PP2A-B56α knockout mice is normalized by β-adrenergic stimulation

The activity of protein phosphatase 2A (PP2A) is determined by the expression and localization of the regulatory B-subunits. PP2A-B56α is the dominant isoform of the B′-family in the heart. Its role in regulating the cardiac response to β-adrenergic stimulation is not yet fully understood. We therefore generated mice deficient in B56α to test the functional cardiac effects in response to catecholamine administration versus corresponding WT mice. We found the decrease in basal PP2A activity in hearts of KO mice was accompanied by a counter-regulatory increase in the expression of B′ subunits (β and γ) and higher phosphorylation of sarcoplasmic reticulum Ca²⁺ regulatory and myofilament proteins. The higher phosphorylation levels were associated with enhanced intraventricular pressure and relaxation in catheterized KO mice. In contrast, at the cellular level, we detected depressed Ca²⁺ transient and sarcomere shortening parameters in KO mice at basal conditions. Consistently, the peak amplitude of the L-type Ca²⁺ current was reduced and the inactivation kinetics of I_CaL were prolonged in KO cardiomyocytes. However, we show β-adrenergic stimulation resulted in a comparable peak amplitude of Ca²⁺ transients and myocellular contraction between KO and WT cardiomyocytes. Therefore, we propose higher isoprenaline-induced Ca²⁺ spark frequencies might facilitate the normalized Ca²⁺ signaling in KO cardiomyocytes. In addition, the application of isoprenaline was associated with unchanged L-type Ca²⁺ current parameters between both groups. Our data suggest an important influence of PP2A-B56α on the regulation of Ca²⁺ signaling and contractility in response to β-adrenergic stimulation in the myocardium.

P1595Increased in vivo perpetuation of whole-heart ventricular arrhythmia in heterozygous Na+/Ca2+-exchanger knockout mice

Background

Commonly, innovative antiarrhythmic strategies are derived from single cell studies that frequently yield promising in vitro findings. However, these results may differ on the whole-heart level, since multicellular electrophysiology is characterized by several emergent features. In previous cellular studies, we have identified the Na+/Ca2+-exchanger (NCX) as a promising target for an innovative antiarrhythmic strategy, as NCX upregulation is present in major cardiac diseases (e.g. heart failure) and promotes independently cellular early and late afterdepolarizations (EADs and DADs). Vice versa, we found that genetic and pharmacological NCX inhibition protects against EADs and DADs. To date, it is unknown, whether the concept of NCX inhibition indeed beneficially applies to the whole-heart level. Thus, we here investigate the in vivo inducibility and perpetuation of whole-heart arrhythmia using a heterozygous NCX-knockout mouse (KO) model that is protected against EADs and DADs on the cellular level.

Methods/Results

Programmed electrical right ventricular stimulation (PVS) and burst stimulation were performed in KO (n=22) and wild-type (n=34) mice by an octapolar mouse electrophysiological catheter introduced via the right jugular vein. Inducibility for ventricular tachycardia (VT) during PVS was similar in WT (73.5%) compared to KO (90.0%) (p=0.1707). With burst stimulation, VT inducibility was higher in KO (KO: 68.2%; WT: 32.4%; p=0.0134). During PVS, KO exhibited increased VT perpetuation as reflected in a significantly prolonged mean (in s; KO: 0.89±0.93; WT: 0.39±0.41; p=0.0097) and cumulative VT duration (in s; KO: 19.54±27.98; WT: 4.46±6.35; p=0.0019). Analysis of animals that were inducible for VT consistently yielded similar results. The ventricular refractory period (VRP) (in ms; KO: 15.1±3.5; WT: 18.7±4.1; p=0.0050) and the QTc interval were shortened in KO (in ms; KO: 46.5±5.8; WT: 53.2±5.9; p=0.0001).

Conclusions

As opposed to findings on the single cell level, KO mice exhibited an increased in vivo arrhythmia burden on the whole-heart level during PVS. This mainly resulted from increased perpetuation of artificially induced VTs, since the inducibility of VTs was not significantly increased in KO with PVS. As a mechanistic explanation of these surprising results, we found significantly reduced VRP and QTc durations in KO in line with the previously demonstrated action potential shortening in single KO cardiomyocytes, which promotes the perpetuation of VTs. We conclude that genetic NCX inhibition can protect from proarrhythmic cellular triggers like EADs and DADs that can initiate VT. However, VTs may perpetuate longer in KO most likely due to reduced refractory periods. This finding carries important translational limitations for the antiarrhythmic concept of NCX inhibition and demonstrates that the value of novel innovative strategies needs evaluation on both the cellular and the whole-heart level.

Acknowledgement/Funding

None

Distinct Occurrence of Proarrhythmic Afterdepolarizations in Atrial Versus Ventricular Cardiomyocytes: Implications for Translational Research on Atrial Arrhythmia

Background: Principal mechanisms of arrhythmia have been derived from ventricular but not atrial cardiomyocytes of animal models despite higher prevalence of atrial arrhythmia (e.g., atrial fibrillation). Due to significant ultrastructural and functional differences, a simple transfer of ventricular proneness toward arrhythmia to atrial arrhythmia is critical. The use of murine models in arrhythmia research is widespread, despite known translational limitations. We here directly compare atrial and ventricular mechanisms of arrhythmia to identify critical differences that should be considered in murine models for development of antiarrhythmic strategies for atrial arrhythmia.

Methods and Results: Isolated murine atrial and ventricular myocytes were analyzed by wide field microscopy and subjected to a proarrhythmic protocol during patch-clamp experiments. As expected, the spindle shaped atrial myocytes showed decreased cell area and membrane capacitance compared to the rectangular shaped ventricular myocytes. Though delayed afterdepolarizations (DADs) could be evoked in a similar fraction of both cell types (80% of cells each), these led significantly more often to the occurrence of spontaneous action potentials (sAPs) in ventricular myocytes. Interestingly, numerous early afterdepolarizations (EADs) were observed in the majority of ventricular myocytes, but there was no EAD in any atrial myocyte (EADs per cell; atrial myocytes: 0 ± 0; n = 25/12 animals; ventricular myocytes: 1.5 [0–43]; n = 20/12 animals; p < 0.05). At the same time, the action potential duration to 90% decay (APD₉₀) was unaltered and the APD₅₀ even increased in atrial versus ventricular myocytes. However, the depolarizing L-type Ca²⁺ current (I_Ca) and Na⁺/Ca²⁺-exchanger inward current (I_NCX) were significantly smaller in atrial versus ventricular myocytes.

Conclusion: In mice, atrial myocytes exhibit a substantially distinct occurrence of proarrhythmic afterdepolarizations compared to ventricular myocytes, since they are in a similar manner susceptible to DADs but interestingly seem to be protected against EADs and show less sAPs. Key factors in the generation of EADs like I_Ca and I_NCX were significantly reduced in atrial versus ventricular myocytes, which may offer a mechanistic explanation for the observed protection against EADs. These findings may be of relevance for current studies on atrial level in murine models to develop targeted strategies for the treatment of atrial arrhythmia.

Overexpression of the Na+ /Ca2+ exchanger influences ouabain-mediated spontaneous Ca2+ activity but not positive inotropy

Administration of digitalis in heart failure (HF) increases quality of life but does not carry a prognostic benefit. Digitalis is an indirect inhibitor of the Na⁺ /Ca²⁺ exchanger (NCX), which is overexpressed in HF. We therefore used the cardiac glycoside ouabain in Ca²⁺ imaging experiments and patch-clamp experiments in isolated ventricular myocytes from nonfailing transgenic NCX overexpressor mice (OE). In field-stimulated myocytes, ouabain (1-100 μm) increased the amplitude of the Ca²⁺ transient in OE and wild-type (WT) similarly. Ouabain-mediated spontaneous Ca²⁺ -activity was significantly more pronounced in OE compared to WT myocytes at higher concentrations (100 μm). Also, at very high concentrations (1000 μm) of ouabain, the number of cells with hypercontraction leading to cell death was higher in OE. Ouabain (10 μm) shortened the action potential duration in both genotypes. Our findings suggest that the proarrhythmic but not the inotropic effects of cardiac glycosides are enhanced by increased NCX expression. This may offer an explanation for the observed lack of prognostic benefit but increased quality of life in HF, which is accompanied by NCX upregulation.

The Effects of SEA0400 on Ca2+ Transient Amplitude and Proarrhythmia Depend on the Na+/Ca2+ Exchanger Expression Level in Murine Models

Background/Objective: The cardiac Na⁺/Ca²⁺ exchanger (NCX) has been identified as a promising target to counter arrhythmia in previous studies investigating the benefit of NCX inhibition. However, the consequences of NCX inhibition have not been investigated in the setting of altered NCX expression and function, which is essential, since major cardiac diseases (heart failure/atrial fibrillation) exhibit NCX upregulation. Thus, we here investigated the effects of the NCX inhibitor SEA0400 on the Ca²⁺ transient amplitude and on proarrhythmia in homozygous NCX overexpressor (OE) and heterozygous NCX knockout (hetKO) mice compared to corresponding wild-types (WT_OE/WT_hetKO). Methods/Results: Ca²⁺ transients of field-stimulated isolated ventricular cardiomyocytes were recorded with fluo-4-AM or indo-1-AM. SEA0400 (1 μM) significantly reduced NCX forward mode function in all mouse lines. SEA0400 (1 μM) significantly increased the amplitude of field-stimulated Ca²⁺ transients in WT_OE, WT_hetKO, and hetKO, but not in OE (% of basal; OE = 98.7 ± 5.0; WT_OE = 137.8 ± 5.2^*; WT_hetKO = 126.3 ± 6.0^*; hetKO = 140.6 ± 12.8^*; ^*p < 0.05 vs. basal). SEA0400 (1 μM) significantly reduced the number of proarrhythmic spontaneous Ca²⁺ transients (sCR) in OE, but increased it in WT_OE, WT_hetKO and hetKO (sCR per cell; basal/+SEA0400; OE = 12.5/3.7; WT_OE = 0.2/2.4; WT_hetKO = 1.3/8.8; hetKO = 0.2/5.5) and induced Ca²⁺ overload with subsequent cell death in hetKO. Conclusion: The effects of SEA0400 on Ca²⁺ transient amplitude and the occurrence of spontaneous Ca²⁺ transients as a proxy measure for inotropy and cellular proarrhythmia depend on the NCX expression level. The antiarrhythmic effect of SEA0400 in conditions of increased NCX expression promotes the therapeutic concept of NCX inhibition in heart failure/atrial fibrillation. Conversely, in conditions of reduced NCX expression, SEA0400 suppressed the NCX function below a critical level leading to adverse Ca²⁺ accumulation as reflected by an increase in Ca²⁺ transient amplitude, proarrhythmia and cell death. Thus, the remaining NCX function under inhibition may be a critical factor determining the inotropic and antiarrhythmic efficacy of SEA0400.

Suppression of Early and Late Afterdepolarizations by Heterozygous Knockout of the Na+/Ca2+ Exchanger in a Murine Model

BACKGROUND

The Na(+)/Ca(2+) exchanger (NCX) has been implied to cause arrhythmias. To date, information on the role of NCX in arrhythmogenesis derived from models with increased NCX expression, hypertrophy, and heart failure. Furthermore, the exact mechanism by which NCX exerts its potentially proarrhythmic effect, ie, by promoting early afterdepolarization (EAD) or delayed afterdepolarization (DAD) or both, is unknown.

METHODS AND RESULTS

We investigated isolated cardiomyocytes from a murine model with heterozygous knockout of NCX (hetKO) using the patch clamp and Ca(2+) imaging techniques. Action potential duration was shorter in hetKO with IKtot not being increased. The rate of spontaneous Ca(2+) release events and the rate of DADs were unaltered; however, DADs had lower amplitude in hetKO. A DAD triggered a spontaneous action potential significantly less often in hetKO when compared with wild-type. The occurrence of EADs was also drastically reduced in hetKO. ICa activity was reduced in hetKO, an effect that was abolished in the presence of the Ca(2+) buffer BAPTA.

CONCLUSIONS

Genetic suppression of NCX reduces both EADs and DADs. The following molecular mechanisms apply: (1) Although the absolute number of DADs is unaffected, an impaired translation of DADs into spontaneous action potentials results from a reduced DAD amplitude. (2) EADs are reduced in absolute number of occurrence, which is presumably a consequence of shortened action potential duration because of reduced NCX activity but also reduced ICa the latter possibly being caused by a direct modulation of Ca(2+)-dependent ICa inhibition by reduced NCX activity. This is the first study to demonstrate that genetic inhibition of NCX protects against afterdepolarizations and to investigate the underlying mechanisms.