Mouse models of arrhythmogenic cardiovascular disease: challenges and opportunities

Intracardiac electrophysiology to characterize susceptibility to ventricular arrhythmias in murine models

Introduction: Sudden cardiac death (SCD) and ventricular fibrillation are rare but severe complications of many cardiovascular diseases and represent a major health issue worldwide. Although the primary causes are often acute or chronic coronary diseases, genetic conditions, such as inherited channelopathies or non-ischemic cardiomyopathies are leading causes of SCD among the young. However, relevant experimental models to study the underlying mechanisms of arrhythmias and develop new therapies are still needed. The number of genetically engineered mouse models with cardiac phenotype is growing, making electrophysiological studies in mice essential tools to study arrhythmogenicity and arrhythmia mechanisms and to test novel treatments. Recently, intracardiac catheterization via the jugular vein was described to induce and record ventricular arrhythmias in living anesthetized mice. Several strategies have been reported, developed in healthy wild-type animals and based on aggressive right ventricular stimulation. Methods: Here, we report a protocol based on programmed electrical stimulation (PES) performed in clinical practice in patients with cardiac rhythm disorders, adapted to two transgenic mice models of arrhythmia - Brugada syndrome and cardiolaminopathy. Results: We show that this progressive protocol, based on a limited number of right ventricular extrastimuli, enables to reveal different rhythmic phenotypes between control and diseased mice. In this study, we provide detailed information on PES in mice, including catheter positioning, stimulation protocols, intracardiac and surface ECG interpretation and we reveal a higher susceptibility of two mouse lines to experience triggered ventricular arrhythmias, when compared to control mice. Discussion: Overall, this technique allows to characterize arrhythmias and provides results in phenotyping 2 arrhythmogenic-disease murine models.

What to consider for ECG in mice-with special emphasis on telemetry

Genetically or surgically altered mice are commonly used as models of human cardiovascular diseases. Electrocardiography (ECG) is the gold standard to assess cardiac electrophysiology as well as to identify cardiac phenotypes and responses to pharmacological and surgical interventions. A variety of methods are used for mouse ECG acquisition under diverse conditions, making it difficult to compare different results. Non-invasive techniques allow only short-term data acquisition and are prone to stress or anesthesia related changes in cardiac activity. Telemetry offers continuous long-term acquisition of ECG data in conscious freely moving mice in their home cage environment. Additionally, it allows acquiring data 24/7 during different activities, can be combined with different challenges and most telemetry systems collect additional physiological parameters simultaneously. However, telemetry transmitters require surgical implantation, the equipment for data acquisition is relatively expensive and analysis of the vast number of ECG data is challenging and time-consuming. This review highlights the limits of non-invasive methods with respect to telemetry. In particular, primary screening using non-invasive methods can give a first hint; however, subtle cardiac phenotypes might be masked or compensated due to anesthesia and stress during these procedures. In addition, we detail the key differences between the mouse and human ECG. It is crucial to consider these differences when analyzing ECG data in order to properly translate the insights gained from murine models to human conditions.

PITX2 Knockout Induces Key Findings of Electrical Remodeling as Seen in Persistent Atrial Fibrillation

BACKGROUND

Electrical remodeling in human persistent atrial fibrillation is believed to result from rapid electrical activation of the atria, but underlying genetic causes may contribute. Indeed, common gene variants in an enhancer region close to PITX2 (paired-like homeodomain transcription factor 2) are strongly associated with atrial fibrillation, but the mechanism behind this association remains unknown. This study evaluated the consequences of PITX2 deletion (PITX2^-/-) in human induced pluripotent stem cell-derived atrial cardiomyocytes.

METHODS

CRISPR/Cas9 (clustered regularly interspaced short palindromic repeats/clustered regularly interspaced short palindromic repeat-associated 9) was used to delete PITX2 in a healthy human iPSC line that served as isogenic control. Human induced pluripotent stem cell-derived atrial cardiomyocytes were differentiated with unfiltered retinoic acid and cultured in atrial engineered heart tissue. Force and action potential were measured in atrial engineered heart tissues. Single human induced pluripotent stem cell-derived atrial cardiomyocytes were isolated from atrial engineered heart tissue for ion current measurements.

RESULTS

PITX2^-/- atrial engineered heart tissue beats slightly slower than isogenic control without irregularity. Force was lower in PITX2^-/- than in isogenic control (0.053±0.015 versus 0.131±0.017 mN, n=28/3 versus n=28/4, PITX2^-/- versus isogenic control; P<0.0001), accompanied by lower expression of CACNA1C and lower L-type Ca²⁺ current density. Early repolarization was weaker (action potential duration at 20% repolarization; 45.5±13.2 versus 8.6±5.3 ms, n=18/3 versus n=12/4, PITX2^-/- versus isogenic control; P<0.0001), and maximum diastolic potential was more negative (-78.3±3.1 versus -69.7±0.6 mV, n=18/3 versus n=12/4, PITX2^-/- versus isogenic control; P=0.001), despite normal inward rectifier currents (both I_K1 and I_K,ACh) and carbachol-induced shortening of action potential duration.

CONCLUSIONS

Complete PITX2 deficiency in human induced pluripotent stem cell-derived atrial cardiomyocytes recapitulates some findings of electrical remodeling of atrial fibrillation in the absence of fast beating, indicating that these abnormalities could be primary consequences of lower PITX2 levels.

The role of the cardiomyocyte circadian clocks in ion channel regulation and cardiac electrophysiology

Daily variations in cardiac electrophysiology and the incidence for different types of arrhythmias reflect ≈24 h changes in the environment, behaviour and internal circadian rhythms. This article focuses on studies that use animal models to separate the impact that circadian rhythms, as well as changes in the environment and behaviour, have on 24 h rhythms in heart rate and ventricular repolarization. Circadian rhythms are initiated at the cellular level by circadian clocks, transcription-translation feedback loops that cycle with a periodicity of 24 h. Several studies now show that the circadian clock in cardiomyocytes regulates the expression of cardiac ion channels by multiple mechanisms; underlies time-of-day changes in sinoatrial node excitability/intrinsic heart rate; and limits the duration of the ventricular action potential waveform. However, the 24 h rhythms in heart rate and ventricular repolarization are primarily driven by autonomic signalling. A functional role for the cardiomyocyte circadian clock appears to buffer the heart against perturbations. For example, the cardiomyocyte circadian clock limits QT-interval prolongation (especially at slower heart rates), and it may facilitate the realignment of the 24 h rhythm in heart rate to abrupt changes in the light cycle. Additional studies show that modifying rhythmic behaviours (including feeding behaviour) can dramatically impact the 24 h rhythms in heart rate and ventricular repolarization. If these mechanisms are conserved, these studies suggest that targeting endogenous circadian mechanisms in the heart, as well as modifying the timing of certain rhythmic behaviours, could emerge as therapeutic strategies to support heart function against perturbations and regulate 24 h rhythms in cardiac electrophysiology.

Timing of food intake in mice unmasks a role for the cardiomyocyte circadian clock mechanism in limiting QT-interval prolongation

Cardiac electrophysiological studies demonstrate that restricting the feeding of mice to the light cycle (time restricted feeding or TRF) causes a pronounced change in heart rate and ventricular repolarization as measured by the RR- and QT-interval, respectively. TRF slows heart rate and shifts the peak (acrophase) of the day/night rhythms in the RR- and QT-intervals from the light to the dark cycle. This study tested the hypothesis that these changes in cardiac electrophysiology are driven by the cardiomyocyte circadian clock mechanism. We determined the impact that TRF had on RR- and QT-intervals in control mice or mice that had the cardiomyocyte circadian clock mechanism disrupted by inducing the deletion of Bmal1 in adult cardiomyocytes (iCSΔBmal1^-/- mice). In control and iCSΔBmal1^-/- mice, TRF increased the RR-intervals measured during the dark cycle and shifted the acrophase of the day/night rhythm in the RR-interval from the light to the dark cycle. Compared to control mice, TRF caused a larger prolongation of the QT-interval measured from iCSΔBmal1^-/- mice during the dark cycle. The larger QT-interval prolongation in the iCSΔBmal1^-/- mice caused an increased mean and amplitude in the day/night rhythm of the QT-interval. There was not a difference in the TRF-induced shift in the day/night rhythm of the QT-interval measured from control or iCSΔBmal1^-/- mice. We conclude that the cardiomyocyte circadian clock does not drive the changes in heart rate or ventricular repolarization with TRF. However, TRF unmasks an important role for the cardiomyocyte circadian clock to prevent excessive QT-interval prolongation, especially at slow heart rates.

Sarcoplasmic Reticulum Calcium Release Is Required for Arrhythmogenesis in the Mouse

Dysfunctional sarcoplasmic reticulum Ca²⁺ handling is commonly observed in heart failure, and thought to contribute to arrhythmogenesis through several mechanisms. Some time ago we developed a cardiomyocyte-specific inducible SERCA2 knockout mouse, which is remarkable in the degree to which major adaptations to sarcolemmal Ca²⁺ entry and efflux overcome the deficit in SR reuptake to permit relatively normal contractile function. Conventionally, those adaptations would also be expected to dramatically increase arrhythmia susceptibility. However, that susceptibility has never been tested, and it is possible that the very rapid repolarization of the murine action potential (AP) allows for large changes in sarcolemmal Ca²⁺ transport without substantially disrupting electrophysiologic stability. We investigated this hypothesis through telemetric ECG recording in the SERCA2-KO mouse, and patch-clamp electrophysiology, Ca²⁺ imaging, and mathematical modeling of isolated SERCA2-KO myocytes. While the SERCA2-KO animals exhibit major (and unique) electrophysiologic adaptations at both the organ and cell levels, they remain resistant to arrhythmia. A marked increase in peak L-type calcium (I_CaL) current and slowed I_CaL decay elicited pronounced prolongation of initial repolarization, but faster late repolarization normalizes overall AP duration. Early afterdepolarizations were seldom observed in KO animals, and those that were observed exhibited a mechanism intermediate between murine and large mammal dynamical properties. As expected, spontaneous SR Ca²⁺ sparks and waves were virtually absent. Together these findings suggest that intact SR Ca²⁺ handling is an absolute requirement for triggered arrhythmia in the mouse, and that in its absence, dramatic changes to the major inward currents can be resisted by the substantial K⁺ current reserve, even at end-stage disease.

A comparative review on heart ion channels, action potentials and electrocardiogram in rodents and human: extrapolation of experimental insights to clinic

Electrocardiogram (ECG) is a non-invasive valuable diagnostic tool that is used in clinics for investigation and monitoring of heart electrical rhythm/conduction, ischemia/injury of heart, electrolyte disturbances and agents/drugs induced cardiac toxicity. Nowadays using animal models to study heart diseases such as electrical and mechanical disturbance is common. In addition, given to ethical consideration and availability, the use of small rodents has been a top priority for cardiovascular researchers. However, extrapolation of experimental findings from the lab to the clinic needs sufficient basic knowledge of similarities and differences between heart action potential and ECG of rodents and humans in normal and disease conditions. This review compares types of human action potentials, the dominant ion currents during action potential phases, alteration in ion channels activities in channelopathies-induced arrhythmias and the ECG appearance of mouse, rat, guinea pig, rabbit and human. Also, it briefly discusses the responsiveness and alterations in ECG following some interventions such as cardiac injury and arrhythmia induction. Overall, it provides a roadmap for researchers in selecting the best animal model/species whose studies results can be translated into clinical practice. In addition, this study will also be useful to biologists, physiologists, pharmacologists, veterinarians and physicians working in the fields of comparative physiology, pharmacology, toxicology and diseases.

Cardiomyocyte Deletion of Bmal1 Exacerbates QT- and RR-Interval Prolongation in Scn5a +/ΔKPQ Mice

Circadian rhythms are generated by cell autonomous circadian clocks that perform a ubiquitous cellular time-keeping function and cell type-specific functions important for normal physiology. Studies show inducing the deletion of the core circadian clock transcription factor Bmal1 in adult mouse cardiomyocytes disrupts cardiac circadian clock function, cardiac ion channel expression, slows heart rate, and prolongs the QT-interval at slow heart rates. This study determined how inducing the deletion of Bmal1 in adult cardiomyocytes impacted the in vivo electrophysiological phenotype of a knock-in mouse model for the arrhythmogenic long QT syndrome (Scn5a^+/ΔKPQ). Electrocardiographic telemetry showed inducing the deletion of Bmal1 in the cardiomyocytes of mice with or without the ΔKPQ-Scn5a mutation increased the QT-interval at RR-intervals that were ≥130 ms. Inducing the deletion of Bmal1 in the cardiomyocytes of mice with or without the ΔKPQ-Scn5a mutation also increased the day/night rhythm-adjusted mean in the RR-interval, but it did not change the period, phase or amplitude. Compared to mice without the ΔKPQ-Scn5a mutation, mice with the ΔKPQ-Scn5a mutation had reduced heart rate variability (HRV) during the peak of the day/night rhythm in the RR-interval. Inducing the deletion of Bmal1 in cardiomyocytes did not affect HRV in mice without the ΔKPQ-Scn5a mutation, but it did increase HRV in mice with the ΔKPQ-Scn5a mutation. The data demonstrate that deleting Bmal1 in cardiomyocytes exacerbates QT- and RR-interval prolongation in mice with the ΔKPQ-Scn5a mutation.

Translational investigation of electrophysiology in hypertrophic cardiomyopathy

Hypertrophic cardiomyopathy (HCM) patients are at increased risk of ventricular arrhythmias and sudden cardiac death, which can occur even in the absence of structural changes of the heart. HCM mouse models suggest mutations in myofilament components to affect Ca²⁺ homeostasis and thereby favor arrhythmia development. Additionally, some of them show indications of pro-arrhythmic changes in cardiac electrophysiology. In this study, we explored arrhythmia mechanisms in mice carrying a HCM mutation in Mybpc3 (Mybpc3-KI) and tested the translatability of our findings in human engineered heart tissues (EHTs) derived from CRISPR/Cas9-generated homozygous MYBPC3 mutant (MYBPC3hom) in induced pluripotent stem cells (iPSC) and to left ventricular septum samples obtained from HCM patients.

We observed higher arrhythmia susceptibility in contractility measurements of field-stimulated intact cardiomyocytes and ventricular muscle strips as well as in electromyogram recordings of Langendorff-perfused hearts from adult Mybpc3-KI mice than in wild-type (WT) controls. The latter only occurred in homozygous (Hom-KI) but not in heterozygous (Het-KI) mouse hearts. Both Het- and Hom-KI are known to display pro-arrhythmic increased Ca²⁺ myofilament sensitivity as a direct consequence of the mutation. In the electrophysiological characterization of the model, we observed smaller repolarizing K⁺ currents in single cell patch clamp, longer ventricular action potentials in sharp microelectrode recordings and longer ventricular refractory periods in Langendorff-perfused hearts in Hom-KI, but not Het-KI. Interestingly, reduced K⁺ channel subunit transcript levels and prolonged action potentials were already detectable in newborn, pre-hypertrophic Hom-KI mice. Human iPSC-derived MYBPC3hom EHTs, which genetically mimicked the Hom-KI mice, did exhibit lower mutant mRNA and protein levels, lower force, beating frequency and relaxation time, but no significant alteration of the force-Ca²⁺ relation in skinned EHTs. Furthermore, MYBPC3hom EHTs did show higher spontaneous arrhythmic behavior, whereas action potentials measured by sharp microelectrode did not differ to isogenic controls. Action potentials measured in septal myectomy samples did not differ between patients with HCM and patients with aortic stenosis, except for the only sample with a MYBPC3 mutation.

The data demonstrate that increased myofilament Ca²⁺ sensitivity is not sufficient to induce arrhythmias in the Mybpc3-KI mouse model and suggest that reduced K⁺ currents can be a pro-arrhythmic trigger in Hom-KI mice, probably already in early disease stages. However, neither data from EHTs nor from left ventricular samples indicate relevant reduction of K⁺ currents in human HCM. Therefore, our study highlights the species difference between mouse and human and emphasizes the importance of research in human samples and human-like models.

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Sudden cardiac death is threatening hypertrophic cardiomyopathy (HCM) patients.

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Arrhythmia mechanisms are not well understood.

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Mouse HCM models showed relevant reduction in K⁺ currents.

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Human iPSC-EHT model and HCM patient septal myectomies did not display this mechanism.

Ambulatory Electrocardiographic Monitoring and Ectopic Beat Detection in Conscious Mice

Ambulatory electrocardiography (AECG) is a primary diagnostic tool in patients with potential arrhythmic disorders. To study the pathophysiological mechanisms of arrhythmic disorders, mouse models are widely implemented. The use of a technique similar to AECG for mice is thus of great relevance. We have optimized a protocol which allows qualitative, long-term ECG data recording in conscious, freely moving mice. Automated algorithms were developed to efficiently process the large amount of data and calculate the average heart rate (HR), the mean peak-to-peak interval and heart rate variability (HRV) based on peak detection. Ectopic beats are automatically detected based on aberrant peak intervals. As we have incorporated a multiple lead configuration in our ECG set-up, the nature and origin of the suggested ectopic beats can be analyzed in detail. The protocol and analysis tools presented here are promising tools for studies which require detailed, long-term ECG characterization in mouse models with potential arrhythmic disorders.

Animal models of arrhythmia: classic electrophysiology to genetically modified large animals

Arrhythmias are common and contribute substantially to cardiovascular morbidity and mortality. The underlying pathophysiology of arrhythmias is complex and remains incompletely understood, which explains why mostly only symptomatic therapy is available. The evaluation of the complex interplay between various cell types in the heart, including cardiomyocytes from the conduction system and the working myocardium, fibroblasts and cardiac immune cells, remains a major challenge in arrhythmia research because it can be investigated only in vivo. Various animal species have been used, and several disease models have been developed to study arrhythmias. Although every species is useful and might be ideal to study a specific hypothesis, we suggest a practical trio of animal models for future use: mice for genetic investigations, mechanistic evaluations or early studies to identify potential drug targets; rabbits for studies on ion channel function, repolarization or re-entrant arrhythmias; and pigs for preclinical translational studies to validate previous findings. In this Review, we provide a comprehensive overview of different models and currently used species for arrhythmia research, discuss their advantages and disadvantages and provide guidance for researchers who are considering performing in vivo studies.

Calmodulin kinase II regulates atrial myocyte late sodium current, calcium handling, and atrial arrhythmia

BACKGROUND

Atrial fibrillation (AF) is the most common type of arrhythmia. Abnormal atrial myocyte Ca²⁺ handling promotes aberrant membrane excitability and remodeling that are important for atrial arrhythmogenesis. The sequence of molecular events leading to loss of normal atrial myocyte Ca²⁺ homeostasis is not established. Late Na⁺ current (I_Na,L) is increased in atrial myocytes from AF patients together with an increase in activity of Ca²⁺/calmodulin-dependent kinase II (CaMKII).

OBJECTIVE

The purpose of this study was to determine whether CaMKII-dependent phosphorylation at Ser571 on Na_V1.5 increases atrial I_Na,L, leading to aberrant atrial Ca²⁺ cycling, altered electrophysiology, and increased AF risk.

METHODS

Atrial myocyte electrophysiology, Ca²⁺ handling, and arrhythmia susceptibility were studied in wild-type and Scn5a knock-in mice expressing phosphomimetic (S571E) or phosphoresistant (S571A) Na_V1.5 at Ser571.

RESULTS

Atrial myocytes from S571E but not S571A mice displayed an increase in I_Na,L and action potential duration, and with adrenergic stress have increased delayed afterdepolarizations. Frequency of Ca²⁺ sparks and waves was increased in S571E atrial myocytes compared to wild type. S571E mice showed an increase in atrial events induced by adrenergic stress and AF inducibility in vivo. Isolated S571E atria were more susceptible to spontaneous atrial events, which were abrogated by inhibiting sarcoplasmic reticulum Ca²⁺ release, CaMKII, or the Na⁺/Ca²⁺ exchanger. Expression of phospho-Na_V1.5 at Ser571 and autophosphorylated CaMKII were increased in atrial samples from human AF patients.

CONCLUSION

This study identified CaMKII-dependent regulation of Na_V1.5 as an important upstream event in Ca²⁺ handling defects and abnormal impulse generation in the setting of AF.

Non-invasive ECG recording and QT interval correction assessment in anesthetized rats and mice

ABSTRACT: Rats and mice are the most common species used in experimental cardiac electrophysiology studies. Electrocardiogram (ECG) recording shows paramount importance for monitoring arrhythmias and cardiac function in several disease models, including QT syndrome. However, the lack of standardized reference values and QT correction formula for different animal species and lineages represent a challenge for ECG interpretation. The aim of this study is to provide an improved method for ECG recording, establishing reference range values and determine the QT formulas with higher correlation to heart rate (HR). A total of 10 Wistar rats, 10 Swiss mice, 10 C57BL/6 mice and 10 FVB/NJ mice were used in the study. Animals were submitted to anesthesia with isoflurane and ECG recording was performed using a six-channel non-invasive electrocardiograph. QT was corrected using the following formulas: Bazzett, Fridericia, Mitchell, Hodges, Van der Water and Framingham. Normal range values for ECG parameters were established in all animals studied. Pearsons’ correlation defined Hodges formula as the most suitable for QT correction. This study demonstrated an improved method of ECG recording with reference values for Swiss, FVB/NJ, C57BL/6 mice, and Wistar rats. Hodges’ formula was the most effective formula for QT correction in rodents, whereas Bazett’s and Friderica formulas were ineffective for such animals. The present work contributes to arrhythmias investigation in experimental cardiology and may reduce misinterpretations in rodents’ ECG.

Overexpression of the HCN2 channel increases the arrhythmogenicity induced by hypokalemia

Hypokalemia, an abnormally low level of potassium (K⁺), is a electrolyte imbalance that commonly occurs in heart failure patients. Hypokalemia is well known to induce lethal ventricular arrhythmia. However, the effects of hypokalemia in failing hearts that have undergone electrophysiological remodeling, i.e., the reactivation of fetal-type ion channels, remain unexplored. We have examined the effect of hypokalemia in the myocytes of transgenic mice overexpressing the hyperpolarization-activated, cyclic nucleotide-sensitive (HCN) channel in the heart (HCN2-Tg mice). Perfusion with a mild hypokalemic solution containing 3 mM K⁺ induced ectopic ventricular automaticity in 55.0% of HCN2-Tg mouse myocytes. In the remaining HCN2-Tg mouse myocytes, the resting membrane potential (RMP) was more depolarized than that of wild-type myocytes subjected to the same treatment and could also be hyperpolarized by an HCN channel blocker. We conclude that in hypokalemia in our mice model, the HCN2 channel was constitutively activated at the hyperpolarized RMP, thereby destabilizing the electrophysiological activity of ventricular myocytes.

Cardiovascular disease models: A game changing paradigm in drug discovery and screening

Cardiovascular disease is the leading cause of death worldwide. Although investment in drug discovery and development has been sky-rocketing, the number of approved drugs has been declining. Cardiovascular toxicity due to therapeutic drug use claims the highest incidence and severity of adverse drug reactions in late-stage clinical development. Therefore, to address this issue, new, additional, replacement and combinatorial approaches are needed to fill the gap in effective drug discovery and screening. The motivation for developing accurate, predictive models is twofold: first, to study and discover new treatments for cardiac pathologies which are leading in worldwide morbidity and mortality rates; and second, to screen for adverse drug reactions on the heart, a primary risk in drug development. In addition to in vivo animal models, in vitro and in silico models have been recently proposed to mimic the physiological conditions of heart and vasculature. Here, we describe current in vitro, in vivo, and in silico platforms for modelling healthy and pathological cardiac tissues and their advantages and disadvantages for drug screening and discovery applications. We review the pathophysiology and the underlying pathways of different cardiac diseases, as well as the new tools being developed to facilitate their study. We finally suggest a roadmap for employing these non-animal platforms in assessing drug cardiotoxicity and safety.

Investigating the Complex Arrhythmic Phenotype Caused by the Gain-of-Function Mutation KCNQ1-G229D

The congenital long QT syndrome (LQTS) is a cardiac electrophysiological disorder that can cause sudden cardiac death. LQT1 is a subtype of LQTS caused by mutations in KCNQ1, affecting the slow delayed-rectifier potassium current (I Ks), which is essential for cardiac repolarization. Paradoxically, gain-of-function mutations in KCNQ1 have been reported to cause borderline QT prolongation, atrial fibrillation (AF), sinus bradycardia, and sudden death, however, the mechanisms are not well understood. The goal of the study is to investigate the ionic, cellular and tissue mechanisms underlying the complex phenotype of a gain-of-function mutation in KCNQ1, c.686G > A (p.G229D) using computer modeling and simulations informed by in vitro measurements. Previous studies have shown this mutation to cause AF and borderline QT prolongation. We report a clinical description of a family that carry this mutation and that a member of the family died suddenly during sleep at 21 years old. Using patch-clamp experiments, we confirm that KCNQ1-G229D causes a significant gain in channel function. We introduce the effect of the mutation in populations of atrial, ventricular and sinus node (SN) cell models to investigate mechanisms underlying phenotypic variability. In a population of human atrial and ventricular cell models and tissue, the presence of KCNQ1-G229D predominantly shortens atrial action potential duration (APD). However, in a subset of models, KCNQ1-G229D can act to prolong ventricular APD by up to 7% (19 ms) and underlie depolarization abnormalities, which could promote QT prolongation and conduction delays. Interestingly, APD prolongations were predominantly seen at slow pacing cycle lengths (CL > 1,000 ms), which suggests a greater arrhythmic risk during bradycardia, and is consistent with the observed sudden death during sleep. In a population of human SN cell models, the KCNQ1-G229D mutation results in slow/abnormal sinus rhythm, and we identify that a stronger L-type calcium current enables the SN to be more robust to the mutation. In conclusion, our computational modeling experiments provide novel mechanistic explanations for the observed borderline QT prolongation, and predict that KCNQ1-G229D could underlie SN dysfunction and conduction delays. The mechanisms revealed in the study can potentially inform management and treatment of KCNQ1 gain-of-function mutation carriers.

Quantification of Beat-To-Beat Variability of Action Potential Durations in Langendorff-Perfused Mouse Hearts

Background: Beat-to-beat variability in action potential duration (APD) is an intrinsic property of cardiac tissue and is altered in pro-arrhythmic states. However, it has never been examined in mice.

Methods: Left atrial or ventricular monophasic action potentials (MAPs) were recorded from Langendorff-perfused mouse hearts during regular 8 Hz pacing. Time-domain, frequency-domain and non-linear analyses were used to quantify APD variability.

Results: Mean atrial APD (90% repolarization) was 23.5 ± 6.3 ms and standard deviation (SD) was 0.9 ± 0.5 ms (n = 6 hearts). Coefficient of variation (CoV) was 4.0 ± 1.9% and root mean square (RMS) of successive differences in APDs was 0.3 ± 0.2 ms. The peaks for low- and high-frequency were 0.7 ± 0.5 and 2.7 ± 0.9 Hz, respectively, with percentage powers of 39.0 ± 20.5 and 59.3 ± 22.9%. Poincaré plots of APD_n+1 against APD_n revealed ellipsoid shapes. The ratio of the SD along the line-of-identity (SD2) to the SD perpendicular to the line-of-identity (SD1) was 8.28 ± 4.78. Approximate and sample entropy were 0.57 ± 0.12 and 0.57 ± 0.15, respectively. Detrended fluctuation analysis revealed short- and long-term fluctuation slopes of 1.80 ± 0.15 and 0.85 ± 0.29, respectively. When compared to atrial APDs, ventricular APDs were longer (ANOVA, P < 0.05), showed lower mean SD and CoV but similar RMS of successive differences in APDs and showed lower SD2 (P < 0.05). No difference in the remaining parameters was observed.

Conclusion: Beat-to-beat variability in APD is observed in mouse hearts during regular pacing. Atrial MAPs showed greater degree of variability than ventricular MAPs. Non-linear techniques offer further insights on short-term and long-term variability and signal complexity.

Limulus and heart rhythm

Great interest in the comparative physiology of hearts and their functions in Animalia has emerged with classic papers on Limulus polyphemus and mollusks. The recurrent cardiac activity-heart rate-is the most important physiological parameter and when present the kardia (Greek) is vital to the development of entire organs of the organisms in the animal kingdom. Extensive studies devoted to the regulation of cardiac rhythm in invertebrates have revealed that the basics of heart physiology are comparable to mammals. The hearts of invertebrates also beat spontaneously and are supplied with regulatory nerves: either excitatory or inhibitory or both. The distinct nerves and the source of excitation/inhibition at the level of single neurons are described for many invertebrate genera. The vertebrates and a majority of invertebrates have myogenic hearts, whereas the horseshoe crab L. polyphemus and a few other animals have a neurogenic cardiac rhythm. Nevertheless, the myogenic nature of heartbeat is precursor, because the contraction of native and stem-cell-derived cardiomyocytes does occur in the absence of any neural elements. Even in L. polyphemus, the heart rhythm is myogenic at embryonic stages.

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.

Transverse cardiac slicing and optical imaging for analysis of transmural gradients in membrane potential and Ca2+ transients in murine heart

Key points

A robust cardiac slicing approach was developed for optical mapping of transmural gradients in transmembrane potential (V_m) and intracellular Ca²⁺ transient (CaT) of murine heart.

Significant transmural gradients in V_m and CaT were observed in the left ventricle.

Frequency‐dependent action potentials and CaT alternans were observed in all ventricular regions with rapid pacing, with significantly greater incidence in the endocardium than epicardium.

The observations demonstrate the feasibility of our new approach to cardiac slicing for systematic analysis of intrinsic transmural and regional gradients in V_m and CaT.

Abstract

Transmural and regional gradients in membrane potential and Ca²⁺ transient in the murine heart are largely unexplored. Here, we developed and validated a robust approach which combines transverse ultra‐thin cardiac slices and high resolution optical mapping to enable systematic analysis of transmural and regional gradients in transmembrane potential (V_m) and intracellular Ca²⁺ transient (CaT) across the entire murine ventricles. The voltage dye RH237 or Ca²⁺ dye Rhod‐2 AM were loaded through the coronary circulation using a Langendorff perfusion system. Short‐axis slices (300 μm thick) were prepared from the entire ventricles (from the apex to the base) by using a high‐precision vibratome. Action potentials (APs) and CaTs were recorded with optical mapping during steady‐state baseline and rapid pacing. Significant transmural gradients in V_m and CaT were observed in the left ventricle, with longer AP duration (APD₅₀ and APD₇₅) and CaT duration (CaTD₅₀ and CaTD₇₅) in the endocardium compared with that in the epicardium. No significant regional gradients were observed along the apico‐basal axis of the left ventricle. Interventricular gradients were detected with significantly shorter APD₅₀, APD₇₅ and CaTD₅₀ in the right ventricle compared with left ventricle and ventricular septum. During rapid pacing, AP and CaT alternans were observed in most ventricular regions, with significantly greater incidence in the endocardium in comparison with epicardium. In conclusion, these observations demonstrate the feasibility of our new approach to cardiac slicing for systematic analysis of intrinsic transmural and regional gradients in V_m and CaT in murine ventricular tissue.

A robust cardiac slicing approach was developed for optical mapping of transmural gradients in transmembrane potential (V_m) and intracellular Ca²⁺ transient (CaT) of murine heart.

Significant transmural gradients in V_m and CaT were observed in the left ventricle.

Frequency‐dependent action potentials and CaT alternans were observed in all ventricular regions with rapid pacing, with significantly greater incidence in the endocardium than epicardium.

The observations demonstrate the feasibility of our new approach to cardiac slicing for systematic analysis of intrinsic transmural and regional gradients in V_m and CaT.

Mathematical modeling physiological effects of the overexpression of β2-adrenoceptors in mouse ventricular myocytes

Transgenic (TG) mice overexpressing β₂-adrenergic receptors (β₂-ARs) demonstrate enhanced myocardial function, which manifests in increased basal adenylyl cyclase activity, enhanced atrial contractility, and increased left ventricular function in vivo. To gain insights into the mechanisms of these effects, we developed a comprehensive mathematical model of the mouse ventricular myocyte overexpressing β₂-ARs. We found that most of the β₂-ARs are active in control conditions in TG mice. The simulations describe the dynamics of major signaling molecules in different subcellular compartments, increased basal adenylyl cyclase activity, modifications of action potential shape and duration, and the effects on L-type Ca²⁺ current and intracellular Ca²⁺ concentration ([Ca²⁺]_i) transients upon stimulation of β₂-ARs in control, after the application of pertussis toxin, upon stimulation with a specific β₂-AR agonist zinterol, and upon stimulation with zinterol in the presence of pertussis toxin. The model also describes the effects of the β₂-AR inverse agonist ICI-118,551 on adenylyl cyclase activity, action potential, and [Ca²⁺]_i transients. The simulation results were compared with experimental data obtained in ventricular myocytes from TG mice overexpressing β₂-ARs and with simulation data on wild-type mice. In conclusion, a new comprehensive mathematical model was developed that describes multiple experimental data on TG mice overexpressing β₂-ARs and can be used to test numerous hypotheses. As an example, using the developed model, we proved the hypothesis of the major contribution of L-type Ca²⁺ current to the changes in the action potential and [Ca²⁺]_i transient upon stimulation of β₂-ARs with zinterol. NEW & NOTEWORTHY We developed a new mathematical model for transgenic mouse ventricular myocytes overexpressing β₂-adrenoceptors that describes the experimental findings in transgenic mice. The model reveals mechanisms of the differential effects of stimulation of β₂-adrenoceptors in wild-type and transgenic mice overexpressing β₂-adrenoceptors.

Bursting dynamics in the normal and failing hearts

A failing heart differs from healthy hearts by an array of symptomatic characteristics, including impaired Ca²⁺ transients, upregulation of Na⁺/Ca²⁺ exchanger function, reduction of Ca²⁺ uptake to sarcoplasmic reticulum, reduced K⁺ currents, and increased propensity to arrhythmias. While significant efforts have been made in both experimental studies and model development to display the causes of heart failure, the full process of deterioration from a healthy to a failing heart yet remains deficiently understood. In this paper, we analyze a highly detailed mathematical model of mouse ventricular myocytes to disclose the key mechanisms underlying the continual transition towards a state of heart failure. We argue that such a transition can be described in mathematical terms as a sequence of bifurcations that the healthy cells undergo while transforming into failing cells. They include normal action potentials and [Ca²⁺]_i transients, action potential and [Ca²⁺]_i alternans, and bursting behaviors. These behaviors where supported by experimental studies of heart failure. The analysis of this model allowed us to identify that the slow component of the fast Na⁺ current is a key determining factor for the onset of bursting activity in mouse ventricular myocytes.

Deletion of Kvβ1.1 subunit leads to electrical and haemodynamic changes causing cardiac hypertrophy in female murine hearts

NEW FINDINGS

What is the central question of this study? The goal of this study was to evaluate sex differences and the role of the potassium channel β1 (Kvβ1) subunit in the heart. What is the main finding and its importance? Genetic ablation of Kvβ1.1 in females led to cardiac hypertrophy characterized by increased heart size, prolonged monophasic action potentials, elevated blood pressure and increased myosin heavy chain α (MHCα) expression. In contrast, male mice showed only electrical changes. Kvβ1.1 binds the MHCα isoform at the protein level, and small interfering RNA targeted knockdown of Kvβ1.1 upregulated MHCα. Cardiovascular disease is the leading cause of death and debility in women in the USA, and cardiac arrhythmias are a major concern. Voltage-gated potassium (Kv) channels along with the binding partners; Kvβ subunits are major regulators of the action potential (AP) shape and duration (APD). The regulation of Kv channels by the Kvβ1 subunit is unknown in female hearts. In the present study, we hypothesized that the Kvβ1 subunit is an important regulator of female cardiac physiology. To test this hypothesis, we ablated (knocked out; KO) the KCNAB1 isoform 1 (Kvβ1.1) subunit in mice and evaluated cardiac function and electrical activity by using ECG, monophasic action potential recordings and echocardiography. Our results showed that the female Kvβ1.1 KO mice developed cardiac hypertrophy, and the hearts were structurally different, with enlargement and increased area. The electrical derangements caused by Kvβ1.1 KO in female mice included long QTc and QRS intervals along with increased APD (APD20-90% repolarization). The male Kvβ1.1 KO mice did not develop cardiac hypertrophy, but they showed long QTc and prolonged APD. Molecular analysis showed that several genes that support cardiac hypertrophy were significantly altered in Kvβ1.1 KO female hearts. In particular, myosin heavy chain α expression was significantly elevated in Kvβ1.1 KO mouse heart. Using a small interfering RNA strategy, we identified that knockdown of Kvβ1 increases myosin heavy chain α expression in H9C2 cells. Collectively, changes in molecular and cell signalling pathways clearly point towards a distinct electrical and structural remodelling consistent with cardiac hypertrophy in the Kvβ1.1 KO female mice.

Genetically engineered livestock for biomedical models

To commemorate Transgenic Animal Research Conference X, this review summarizes the recent progress in developing genetically engineered livestock species as biomedical models. The first of these conferences was held in 1997, which turned out to be a watershed year for the field, with two significant events occurring. One was the publication of the first transgenic livestock animal disease model, a pig with retinitis pigmentosa. Before that, the use of livestock species in biomedical research had been limited to wild-type animals or disease models that had been induced or were naturally occurring. The second event was the report of Dolly, a cloned sheep produced by somatic cell nuclear transfer. Cloning subsequently became an essential part of the process for most of the models developed in the last 18 years and is stilled used prominently today. This review is intended to highlight the biomedical modeling achievements that followed those key events, many of which were first reported at one of the previous nine Transgenic Animal Research Conferences. Also discussed are the practical challenges of utilizing livestock disease models now that the technical hurdles of model development have been largely overcome.

Ion channelopathies in human induced pluripotent stem cell derived cardiomyocytes: a dynamic clamp study with virtual IK1

Human induced pluripotent stem cell derived cardiomyocytes (hiPSC-CMs) are widely used in studying basic mechanisms of cardiac arrhythmias that are caused by ion channelopathies. Unfortunately, the action potential profile of hiPSC-CMs-and consequently the profile of individual membrane currents active during that action potential-differs substantially from that of native human cardiomyocytes, largely due to almost negligible expression of the inward rectifier potassium current (IK1). In the present study, we attempted to "normalize" the action potential profile of our hiPSC-CMs by inserting a voltage dependent in silico IK1 into our hiPSC-CMs, using the dynamic clamp configuration of the patch clamp technique. Recordings were made from single hiPSC-CMs, using the perforated patch clamp technique at physiological temperature. We assessed three different models of IK1, with different degrees of inward rectification, and systematically varied the magnitude of the inserted IK1. Also, we modified the inserted IK1 in order to assess the effects of loss- and gain-of-function mutations in the KCNJ2 gene, which encodes the Kir2.1 protein that is primarily responsible for the IK1 channel in human ventricle. For our experiments, we selected spontaneously beating hiPSC-CMs, with negligible IK1 as demonstrated in separate voltage clamp experiments, which were paced at 1 Hz. Upon addition of in silico IK1 with a peak outward density of 4-6 pA/pF, these hiPSC-CMs showed a ventricular-like action potential morphology with a stable resting membrane potential near -80 mV and a maximum upstroke velocity >150 V/s (n = 9). Proarrhythmic action potential changes were observed upon injection of both loss-of-function and gain-of-function IK1, as associated with Andersen-Tawil syndrome type 1 and short QT syndrome type 3, respectively (n = 6). We conclude that injection of in silico IK1 makes the hiPSC-CM a more reliable model for investigating mechanisms underlying cardiac arrhythmias.

Targeting protein tyrosine phosphatase σ after myocardial infarction restores cardiac sympathetic innervation and prevents arrhythmias

Millions of people suffer a myocardial infarction (MI) every year, and those who survive have increased risk of arrhythmias and sudden cardiac death. Recent clinical studies have identified sympathetic denervation as a predictor of increased arrhythmia susceptibility. Chondroitin sulfate proteoglycans present in the cardiac scar after MI prevent sympathetic reinnervation by binding the neuronal protein tyrosine phosphatase receptor σ (PTPσ). Here we show that the absence of PTPσ, or pharmacologic modulation of PTPσ by the novel intracellular sigma peptide (ISP) beginning 3 days after injury, restores sympathetic innervation to the scar and markedly reduces arrhythmia susceptibility. Using optical mapping we observe increased dispersion of action potential duration, supersensitivity to β-adrenergic receptor stimulation and Ca(2+) mishandling following MI. Sympathetic reinnervation prevents these changes and renders hearts remarkably resistant to induced arrhythmias.