Transgenic rabbit models for cardiac disease research.

To study the pathophysiology of human cardiac diseases and to develop novel treatment strategies, complex interactions of cardiac cells on cellular, tissue and whole heart levels need to be considered. As in vitro cell-based models do not depict the complexity of the human heart, animal models are used to obtain insights that can be translated to human diseases. Mice are the most commonly used animals in cardiac research, however, differences in electrophysiological and mechanical cardiac function and a different composition of electrical and contractile proteins limit the transferability of the knowledge gained. Moreover, the small heart size and fast heart rate are major disadvantages. In contrast to rodents, electrophysiological, mechanical, and structural cardiac characteristics of rabbits resemble the human heart more closely, making them particularly suitable as an animal model for cardiac disease research. In this review, various methodological approaches for the generation of transgenic rabbits for cardiac disease research - such as pronuclear microinjection, the sleeping beauty transposon system and novel genome editing methods (ZFN and CRISPR/Cas9) - will be discussed. In the second section, we will introduce the different currently available transgenic rabbit models for monogenic cardiac diseases (such as long-QT syndrome, short-QT syndrome, and hypertrophic cardiomyopathy) in detail, especially in regards to their utility to increase the understanding of pathophysiological disease-mechanisms and novel treatment options

INVESTIGATION OF CARDIAC ELECTROPHYSIOLOGY IN HUMAN VENTRICULAR TISSUE

Individuals with cardiomyopathy are at higher risk to die from sudden cardiac arrest than those with non-failing (NF) hearts. This study examined the differences in electrical properties of failing and NF human hearts in terms of cardiac memory through explicit control of diastolic intervals in a sinusoidal fashion, restitution of action potential duration (APD) through standard and dynamic pacing protocols, maximum rate of depolarization and APD alternans. Recordings of transmembrane potentials were made in tissues extracted from patients with heart failure and one donor NF heart. Computational simulations were performed using the O’Hara Rudy model for generating surrogates of control data. Significant differences were seen between left ventricular (LV) tissue and NF LV tissue in tilt, and measures of memory in terms of area and thickness during the sinusoidal 400ms protocol. Minimum delay was also significantly higher in the failing LV during the sinusoidal 150ms protocol. Failing tissues showed a higher restitution slope and prolonged AP which is consistent with previous studies and is hypothesized to contribute to the increased susceptibility to unstable alternans. This study further explored how disease alters the electrical functioning of the heart and why these patients are at a higher risk of ventricular arrhythmia

The Role of Electrocardiographic Markers in the Prevention of Atrial and Ventricular Arrhythmias

In our chapter, we overview the main clinical conditions that increase arrhythmogenicity, and we present the surface electrocardiogram (ECG) markers that could be suitable for the prediction of atrial and ventricular arrhythmias. We highlight the clinical value of the prolongation of the P-wave duration and P dispersion (Pd) in the prediction of atrial fibrillation, and we also expound the utility of QT interval, T-wave peak-to-end interval (Tpe), and Tpe/QT ratio (known as arrhythmogenic index (AIX)) in the prediction of ventricular arrhythmias. Furthermore, we present the results of our clinical investigations with regard to surface ECG markers among patients with increased arrhythmia vulnerability. Moreover, we mention other, novel, effectively used ECG markers

Techniques for ventricular repolarization instability assessment from the ECG

Instabilities in ventricular repolarization have been documented to be tightly linked to arrhythmia vulnera- bility. Translation of the information contained in the repolar- ization phase of the electrocardiogram (ECG) into valuable clinical decision-making tools remains challenging. This work aims at providing an overview of the last advances in the pro- posal and quantification of ECG-derived indices that describe repolarization properties and whose alterations are related with threatening arrhythmogenic conditions. A review of the state of the art is provided, spanning from the electrophysio- logical basis of ventricular repolarization to its characteriza- tion on the surface ECG through a set of temporal and spatial risk markers

Blockade of sodium‑calcium exchanger via ORM-10962 attenuates cardiac alternans

Repolarization alternans, a periodic oscillation of long-short action potential duration, is an important source of arrhythmogenic substrate, although the mechanisms driving it are insufficiently understood. Despite its relevance as an arrhythmia precursor, there are no successful therapies able to target it specifically. We hypothesized that blockade of the sodium‑calcium exchanger (NCX) could inhibit alternans. The effects of the selective NCX blocker ORM-10962 were evaluated on action potentials measured with microelectrodes from canine papillary muscle preparations, and calcium transients measured using Fluo4-AM from isolated ventricular myocytes paced to evoke alternans. Computer simulations were used to obtain insight into the drug's mechanisms of action. ORM-10962 attenuated cardiac alternans, both in action potential duration and calcium transient amplitude. Three morphological types of alternans were observed, with differential response to ORM-10962 with regards to APD alternans attenuation. Analysis of APD restitution indicates that calcium oscillations underlie alternans formation. Furthermore, ORM-10962 did not markedly alter APD restitution, but increased post-repolarization refractoriness, which may be mediated by indirectly reduced L-type calcium current. Computer simulations reproduced alternans attenuation via ORM-10962, suggesting that it is acts by reducing sarcoplasmic reticulum release refractoriness. This results from the ORM-10962-induced sodium‑calcium exchanger block accompanied by an indirect reduction in L-type calcium current. Using a computer model of a heart failure cell, we furthermore demonstrate that the anti-alternans effect holds also for this disease, in which the risk of alternans is elevated. Targeting NCX may therefore be a useful anti-arrhythmic strategy to specifically prevent calcium driven alternans

Calcium buffering in the heart in health and disease

Changes of intracellular Ca2+ concentration regulate many aspects of cardiac myocyte function. About 99% of the cytoplasmic calcium in cardiac myocytes is bound to buffers, and their properties will therefore have a major influence on Ca2+ signaling. This article considers the fundamental properties and identities of the buffers and how to measure them. It reviews the effects of buffering on the systolic Ca2+ transient and how this may change physiologically, and in heart failure and both atrial and ventricular arrhythmias, as well. It is concluded that the consequences of this strong buffering may be more significant than currently appreciated, and a fuller understanding is needed for proper understanding of cardiac calcium cycling and contractility

Microvolt T-Wave Alternans Physiological Basis, Methods of Measurement, and Clinical Utility—Consensus Guideline by International Society for Holter and Noninvasive Electrocardiology

This consensus guideline was prepared on behalf of the International Society for Holter and Noninvasive Electrocardiology and is cosponsored by the Japanese Circulation Society, the Computers in Cardiology Working Group on e-Cardiology of the European Society of Cardiology, and the European Cardiac Arrhythmia Society. It discusses the electrocardiographic phenomenon of T-wave alternans (TWA) (i.e., a beat-to-beat alternation in the morphology and amplitude of the ST- segment or T-wave). This statement focuses on its physiological basis and measurement technologies and its clinical utility in stratifying risk for life-threatening ventricular arrhythmias. Signal processing techniques including the frequency-domain Spectral Method and the time-domain Modified Moving Average method have demonstrated the utility of TWA in arrhythmia risk stratification in prospective studies in >12,000 patients. The majority of exercise-based studies using both methods have reported high relative risks for cardiovascular mortality and for sudden cardiac death in patients with preserved as well as depressed left ventricular ejection fraction. Studies with ambulatory electrocardiogram-based TWA analysis with Modified Moving Average method have yielded significant predictive capacity. However, negative studies with the Spectral Method have also appeared, including 2 interventional studies in patients with implantable defibrillators. Meta-analyses have been performed to gain insights into this issue. Frontiers of TWA research include use in arrhythmia risk stratification of individuals with preserved ejection fraction, improvements in predictivity with quantitative analysis, and utility in guiding medical as well as device-based therapy. Overall, although TWA appears to be a useful marker of risk for arrhythmic and cardiovascular death, there is as yet no definitive evidence that it can guide therapy

Real-time optical manipulation of cardiac conduction in intact hearts

Optogenetics has provided new insights in cardiovascular research, leading to new methods for cardiac pacing, resynchronization therapy and cardioversion. Although these interventions have clearly demonstrated the feasibility of cardiac manipulation, current optical stimulation strategies do not take into account cardiac wave dynamics in real time. Here, we developed an all‐optical platform complemented by integrated, newly developed software to monitor and control electrical activity in intact mouse hearts. The system combined a wide‐field mesoscope with a digital projector for optogenetic activation. Cardiac functionality could be manipulated either in free‐run mode with submillisecond temporal resolution or in a closed‐loop fashion: a tailored hardware and software platform allowed real‐time intervention capable of reacting within 2 ms. The methodology was applied to restore normal electrical activity after atrioventricular block, by triggering the ventricle in response to optically mapped atrial activity with appropriate timing. Real‐time intraventricular manipulation of the propagating electrical wavefront was also demonstrated, opening the prospect for real‐time resynchronization therapy and cardiac defibrillation. Furthermore, the closed‐loop approach was applied to simulate a re‐entrant circuit across the ventricle demonstrating the capability of our system to manipulate heart conduction with high versatility even in arrhythmogenic conditions. The development of this innovative optical methodology provides the first proof‐of‐concept that a real‐time optically based stimulation can control cardiac rhythm in normal and abnormal conditions, promising a new approach for the investigation of the (patho)physiology of the heart

MYOFILAMENT DYNAMICS MODULATE CELLULAR DRIVERS OF ARRHYTHMOGENESIS IN HUMAN CARDIAC DISEASE

Cardiac arrhythmia is common in cardiac disease, but our understanding of the cellular mechanisms underlying arrhythmia is incomplete. Alternans, the beat-to- beat alteration in cardiac electrical or mechanical signals, has been linked to susceptibility to lethal arrhythmias in human heart failure (HF) as well as in human chronic atrial fibrillation (cAF). In addition, early afterdepolarizations (EADs), a type of aberrant cellular behavior that causes spontaneous slowing or reversal of normal repolarization, have been implicated as an arrhythmogenic trigger in human hypertrophic cardiomyopathy (HCM). Abnormal myofilament dynamics has been observed in human HF, HCM, and cAF, but whether this aberrant behavior alters the formation of alternans or EADs remains unknown. To address this gap in understanding, a computational modeling approach was taken. Three mechanistically-based bidirectionally coupled human electromechanical myocyte models were constructed, under the conditions of HF, HCM, or cAF. Our goal was to elucidate whether aberrant myofilament dynamics modulate the cellular drivers of arrhythmogenesis in human cardiac disease. In simulations with our human HF ventricular myocyte model, we found that the magnitudes of force, calcium, and action potential voltage alternans were modulated by heart failure induced-remodeling of mechanical parameters and sarcomere length due to the presence of myofilament feedback (MEF) at clinically-relevant pacing rates. In simulations with our human HCM ventricular myocyte model, we found that incorporating MEF diminished the degree of repolarization reserve reduction necessary for EADs to emerge and increased the frequency of EAD occurrence, especially at faster pacing rates. Longer sarcomere lengths and HCM-induced enhanced thin filament activation diminished the effects of MEF on EADs. Finally, in simulations with our human cAF atrial myocyte model, we found that MEF, via cooperativity in Troponin C buffering of cytoplasmic Ca 2+ , diminished the magnitude of action potential duration alternans. Also, enhanced thin filament activation, via either cAF-induced myofilament remodeling or longer sarcomere lengths, enhanced action potential duration alternans. Together, this thesis provides evidence that myofilament protein dynamics mechanisms play an important role in EAD and alternans formation, suggesting that targeting MEF or reversing disease induced myofilament remodeling may be alternative treatment approaches for prevention of arrhythmia in cAF patients