Biological pacemakers: Ready for the clinic?

Local tissue mechanics control cardiac pacemaker cell embryonic patterning

Cardiac pacemaker cells (CPCs) initiate the electric impulses that drive the rhythmic beating of the heart. CPCs reside in a heterogeneous, ECM-rich microenvironment termed the sinoatrial node (SAN). Surprisingly, little is known regarding the biochemical composition or mechanical properties of the SAN, and how the unique structural characteristics present in this region of the heart influence CPC function remains poorly understood. Here, we have identified that SAN development involves the construction of a "soft" macromolecular ECM that specifically encapsulates CPCs. In addition, we demonstrate that subjecting embryonic CPCs to substrate stiffnesses higher than those measured in vivo results in loss of coherent electrical oscillation and dysregulation of the HCN4 and NCX1 ion channels required for CPC automaticity. Collectively, these data indicate that local mechanics play a critical role in maintaining the embryonic CPC function while also quantitatively defining the range of material properties that are optimal for embryonic CPC maturation.

Assembly of the Cardiac Pacemaking Complex: Electrogenic Principles of Sinoatrial Node Morphogenesis

Cardiac pacemaker cells located in the sinoatrial node initiate the electrical impulses that drive rhythmic contraction of the heart. The sinoatrial node accounts for only a small proportion of the total mass of the heart yet must produce a stimulus of sufficient strength to stimulate the entire volume of downstream cardiac tissue. This requires balancing a delicate set of electrical interactions both within the sinoatrial node and with the downstream working myocardium. Understanding the fundamental features of these interactions is critical for defining vulnerabilities that arise in human arrhythmic disease and may provide insight towards the design and implementation of the next generation of potential cellular-based cardiac therapeutics. Here, we discuss physiological conditions that influence electrical impulse generation and propagation in the sinoatrial node and describe developmental events that construct the tissue-level architecture that appears necessary for sinoatrial node function.

Gene and Cell Therapy for Cardiac Arrhythmias

PURPOSE

In the last decade, interest in gene therapy as a therapeutic technology has increased, largely driven by an exciting yet modest number of successful applications for monogenic diseases. Setbacks in the use of gene therapy for cardiac disease have motivated efforts to develop vectors with enhanced tropism for the heart and more efficient delivery methods. Although monogenic diseases are the logical target, cardiac arrhythmias represent a group of conditions amenable to gene therapy because of focal targets (biological pacemakers, nodal conduction, or stem cell-related arrhythmias) or bystander effects on cells not directly transduced because of electrical coupling.

METHODS

This review provides a contemporary narrative of the field of gene therapy for experimental cardiac arrhythmias, including those associated with stem cell transplant. Recent articles published in the English language and available through the PubMed database and other prominent literature are discussed.

FINDINGS

The promise of gene therapy has been realized for a handful of monogenic diseases and is actively being pursued for cardiac applications in preclinical models. With improved vectors, it is likely that cardiac disease will also benefit from this technology. Cardiac arrhythmias, whether inherited or acquired, are a group of conditions with a potentially lower threshold for phenotypic correction and as such hold unique potential as targets for cardiac gene therapy.

IMPLICATIONS

There has been a proliferation of research on the potential of gene therapy for cardiac arrhythmias. This body of investigation forms a strong basis on which further developments, particularly with viral vectors, are likely to help this technology progress along its translational trajectory.

The Quantitative Relationship among the Number of the Pacing Cells Required, the Dimension, and the Diffusion Coefficient

The purpose of the paper is to derive a formula to describe the quantitative relationship among the number of the pacing cells required (NPR), the dimension i, and the diffusion coefficient D (electrical coupling or gap junction G). The relationship between NPR and G has been investigated in different dimensions, respectively. That is, for each fixed i, there is a formula to describe the relationship between NPR and G; and three formulas are required for the three dimensions. However, there is not a universal expression to describe the relationship among NPR, G, and i together. In the manuscript, surveying and investigating the basic law among the existed data, we speculate the preliminary formula of the relationship among the NPR, i, and G; and then, employing the cftool in MATLAB, the explicit formulas are derived for different cases. In addition, the goodness of fit (R ²) is computed to evaluate the fitting of the formulas. Moreover, the 1D and 2D ventricular tissue models containing biological pacemakers are developed to derive more data to validate the formula. The results suggest that the relationship among the NPR, i, and the G (D) could be described by a universal formula, where the NPR scales with the i (the dimension) power of the product of the square root of G (D) and a constant b which is dependent on the strength of the pacing cells and so on.

A Simulation Study on the Pacing and Driving of the Biological Pacemaker

The research on the biological pacemaker has been very active in recent years. And turning nonautomatic ventricular cells into pacemaking cells is believed to hold the key to making a biological pacemaker. In the study, the inward-rectifier K+ current (I K1) is depressed to induce the automaticity of the ventricular myocyte, and then, the effects of the other membrane ion currents on the automaticity are analyzed. It is discovered that the L-type calcium current (I CaL) plays a major part in the rapid depolarization of the action potential (AP). A small enough I CaL would lead to the failure of the automaticity of the ventricular myocyte. Meanwhile, the background sodium current (I bNa), the background calcium current (I bCa), and the Na+/Ca2+ exchanger current (I NaCa) contribute significantly to the slow depolarization, indicating that these currents are the main supplementary power of the pacing induced by depressing I K1, while in the 2D simulation, we find that the weak electrical coupling plays a more important role in the driving of a biological pacemaker.

Genetic Ablation of TASK-1 (Tandem of P Domains in a Weak Inward Rectifying K+ Channel-Related Acid-Sensitive K+ Channel-1) (K2P3.1) K+ Channels Suppresses Atrial Fibrillation and Prevents Electrical Remodeling

BACKGROUND

Despite an increasing understanding of atrial fibrillation (AF) pathophysiology, translation into mechanism-based treatment options is lacking. In atrial cardiomyocytes of patients with chronic AF, expression, and function of tandem of P domains in a weak inward rectifying TASK-1 (K⁺ channel-related acid-sensitive K⁺ channel-1) (K_2P3.1) atrial-specific 2-pore domain potassium channels is enhanced, resulting in action potential duration shortening. TASK-1 channel inhibition prevents action potential duration shortening to maintain values observed among sinus rhythm subjects. The present preclinical study used a porcine AF model to evaluate the antiarrhythmic efficacy of TASK-1 inhibition by adeno-associated viral anti-TASK-1-siRNA (small interfering RNA) gene transfer.

METHODS

AF was induced in domestic pigs by atrial burst stimulation via implanted pacemakers. Adeno-associated viral vectors carrying anti-TASK-1-siRNA were injected into both atria to suppress TASK-1 channel expression. After the 14-day follow-up period, porcine cardiomyocytes were isolated from right and left atrium, followed by electrophysiological and molecular characterization.

RESULTS

AF was associated with increased TASK-1 transcript, protein and ion current levels leading to shortened action potential duration in atrial cardiomyocytes compared to sinus rhythm controls, similar to previous findings in humans. Anti-TASK-1 adeno-associated viral application significantly reduced AF burden in comparison to untreated AF pigs. Antiarrhythmic effects of anti-TASK-1-siRNA were associated with reduction of TASK-1 currents and prolongation of action potential durations in atrial cardiomyocytes to sinus rhythm values. Conclusions Adeno-associated viral-based anti-TASK-1 gene therapy suppressed AF and corrected cellular electrophysiological remodeling in a porcine model of AF. Suppression of AF through selective reduction of TASK-1 currents represents a new option for antiarrhythmic therapy.

Next-generation pacemakers: from small devices to biological pacemakers

Electrogenesis in the heart begins in the sinoatrial node and proceeds down the conduction system to originate the heartbeat. Conduction system disorders lead to slow heart rates that are insufficient to support the circulation, necessitating implantation of electronic pacemakers. The typical electronic pacemaker consists of a subcutaneous generator and battery module attached to one or more endocardial leads. New leadless pacemakers can be implanted directly into the right ventricular apex, providing single-chamber pacing without a subcutaneous generator. Modern pacemakers are generally reliable, and their programmability provides options for different pacing modes tailored to specific clinical needs. Advances in device technology will probably include alternative energy sources and dual-chamber leadless pacing in the not-too-distant future. Although effective, current electronic devices have limitations related to lead or generator malfunction, lack of autonomic responsiveness, undesirable interactions with strong magnetic fields, and device-related infections. Biological pacemakers, generated by somatic gene transfer, cell fusion, or cell transplantation, provide an alternative to electronic devices. Somatic reprogramming strategies, which involve transfer of genes encoding transcription factors to transform working myocardium into a surrogate sinoatrial node, are furthest along in the translational pipeline. Even as electronic pacemakers become smaller and less invasive, biological pacemakers might expand the therapeutic armamentarium for conduction system disorders.

Generation of cardiac pacemaker cells by programming and differentiation

A number of diseases are caused by faulty function of the cardiac pacemaker and described as "sick sinus syndrome". The medical treatment of sick sinus syndrome with electrical pacemaker implants in the diseased heart includes risks. These problems may be overcome via "biological pacemaker" derived from different adult cardiac cells or pluripotent stem cells. The generation of cardiac pacemaker cells requires the understanding of the pacing automaticity. Two characteristic phenomena the "membrane-clock" and the "Ca(2+)-clock" are responsible for the modulation of the pacemaker activity. Processes in the "membrane-clock" generating the spontaneous pacemaker firing are based on the voltage-sensitive membrane ion channel activity starting with slow diastolic depolarization and discharging in the action potential. The influence of the intracellular Ca(2+) modulating the pacemaker activity is characterized by the "Ca(2+)-clock". The generation of pacemaker cells started with the reprogramming of adult cardiac cells by targeted induction of one pacemaker function like HCN1-4 overexpression and enclosed in an activation of single pacemaker specific transcription factors. Reprogramming of adult cardiac cells with the transcription factor Tbx18 created cardiac cells with characteristic features of cardiac pacemaker cells. Another key transcription factor is Tbx3 specifically expressed in the cardiac conduction system including the sinoatrial node and sufficient for the induction of the cardiac pacemaker gene program. For a successful cell therapeutic practice, the generated cells should have all regulating mechanisms of cardiac pacemaker cells. Otherwise, the generated pacemaker cells serve only as investigating model for the fundamental research or as drug testing model for new antiarrhythmics. This article is part of a Special Issue entitled: Cardiomyocyte Biology: Integration of Developmental and Environmental Cues in the Heart edited by Marcus Schaub and Hughes Abriel.