Transplanted Neonatal Cardiomyocytes as a Potential Biological Pacemaker in Pigs with Complete Atrioventricular Block

In silico study of multicellular automaticity of heterogeneous cardiac cell monolayers: Effects of automaticity strength and structural linear anisotropy

The biological pacemaker approach is an alternative to cardiac electronic pacemakers. Its main objective is to create pacemaking activity from added or modified distribution of spontaneous cells in the myocardium. This paper aims to assess how automaticity strength of pacemaker cells (i.e. their ability to maintain robust spontaneous activity with fast rate and to drive neighboring quiescent cells) and structural linear anisotropy, combined with density and spatial distribution of pacemaker cells, may affect the macroscopic behavior of the biological pacemaker. A stochastic algorithm was used to randomly distribute pacemaker cells, with various densities and spatial distributions, in a semi-continuous mathematical model. Simulations of the model showed that stronger automaticity allows onset of spontaneous activity for lower densities and more homogeneous spatial distributions, displayed more central foci, less variability in cycle lengths and synchronization of electrical activation for similar spatial patterns, but more variability in those same variables for dissimilar spatial patterns. Compared to their isotropic counterparts, in silico anisotropic monolayers had less central foci and displayed more variability in cycle lengths and synchronization of electrical activation for both similar and dissimilar spatial patterns. The present study established a link between microscopic structure and macroscopic behavior of the biological pacemaker, and may provide crucial information for optimized biological pacemaker therapies.

Implantation of electronic pacemakers is a standard treatment to pathologically slow heart rhythm. Despite improving quality of life, those devices display many shortcomings. Bioengineered tissue pacemakers may be a therapeutic alternative, but associated design methods usually lack control of the way cells with spontaneous activity are scattered throughout the tissue. Our study is the first to use a mathematical model to rigorously define and thoroughly characterize how pacemaker cells scattering at the microscopic level may affect macroscopic behaviors of the bioengineered tissue pacemaker. Automaticity strength (ability of pacemaker cell to drive its non-pacemaker neighbors) and anisotropy (preferential orientation of cell shape) are also implemented and give unparalleled insights on how effects of uncontrollable scattered pacemaker cells may be modulated by available experimental techniques. Our model is a powerful tool to aid in optimized bioengineered pacemaker therapies.

Effect of mHCN2 gene modification on chronotropic relevant receptors in BMSCs co-cultured with atrial myocytes

Currently, the mechanism of the chronotropic ability of stem cells modified to express the hyperpolarization-activated cyclic nucleotide-gated (HCN) gene remains to be elucidated. The present study assessed the effects of mouse (m)HCN2 gene modification on the expression of chronotropic relevant receptors, adrenergic receptor β1 (Adrb1) and cholinergic receptor muscarinic M2 (Chrm2), in bone marrow stromal cells (BMSCs) co-cultured with atrial myocytes. BMSCs were divided into the following four groups: i) BMSCs transfected with the mHCN2 gene and co-cultured with atrial myocytes for 48 h (TF + CO); ii) respective transfection (TF); iii) respective co-culture (CO); and iv) the control group without treatment (CTL). Green fluorescent protein (GFP) was observed in the BMSCs 48 h after transfection with pEGFP-C1-mHCN2. The expression of Adrb1 and Chrm2 was significantly increased in the TF and TF + CO groups, particularly the TF + CO group, compared with the CTL group (P<0.05). This suggests that BMSCs modified to express the mHCN2 gene possess autorhythmicity and chronotropic ability, particularly when co-cultured with atrial myocytes. The results of the present study provide novel information regarding the molecular basis of biological pacemakers' chronotropic ability.

Noise-induced effects on multicellular biopacemaker spontaneous activity: Differences between weak and strong pacemaker cells

Self-organization of spontaneous activity of a network of active elements is important to the general theory of reaction-diffusion systems as well as for pacemaking activity to initiate beating of the heart. Monolayer cultures of neonatal rat ventricular myocytes, consisting of resting and pacemaker cells, exhibit spontaneous activation of their electrical activity. Similarly, one proposed approach to the development of biopacemakers as an alternative to electronic pacemakers for cardiac therapy is based on heterogeneous cardiac cells with resting and spontaneously beating phenotypes. However, the combined effect of pacemaker characteristics, density, and spatial distribution of the pacemaker cells on spontaneous activity is unknown. Using a simple stochastic pattern formation algorithm, we previously showed a clear nonlinear dependency of spontaneous activity (occurrence and amplitude of spontaneous period) on the spatial patterns of pacemaker cells. In this study, we show that this behavior is dependent on the pacemaker cell characteristics, with weaker pacemaker cells requiring higher density and larger clusters to sustain multicellular activity. These multicellular structures also demonstrated an increased sensitivity to voltage noise that favored spontaneous activity at lower density while increasing temporal variation in the period of activity. This information will help researchers overcome the current limitations of biopacemakers.

Cardiac cell therapy and arrhythmias

Cardiac stem cell based therapy is a promising therapy for patients with severe heart failure. Many types of stem cells, such as embryonic stem cells, myoblasts, marrow-derived mesenchymal stem cells, circulating endothelial progenitor cells, and cardiac precursor cells etc, are known as cellular sources for cardiac stem cell therapy. Both in the clinical and experimental setting, stem cells are reported, and supposed, to cause some arrhythmogenic adverse effects. In order to overcome these serious adverse effects, it is necessary to know the electrophysiological properties of stem cell-derived cardiomyocytes, and have a profound insight into the mechanisms of arrhythmia to know whether such arrhythmogenic properties of the cells can cause serious arrhythmia in situ. In the present study, recent publications that focus on the electrophysiological aspect of stem cell based therapy are reviewed and, furthermore, a new perspective on cardiac stem cell therapy of arrhythmias is given.

Development of an ovine model of pediatric complete heart block

BACKGROUND

Complete heart block is a significant clinical problem that can limit the quality of life in affected children. To understand the pathophysiology of this condition and provide for development of novel therapies, we sought to establish a large animal model of permanent, pacemaker-dependent atrioventricular block (AVB) that mimics the size and growth characteristics of pediatric patients.

MATERIALS AND METHODS

We utilized nine immature lambs weighing 10.5 ± 1.4 kg. After implantation of dual-chamber pacemaker devices with fixed leads, AVB was produced by interrupting His-bundle conduction using radio-frequency ablation at the base of the non-coronary cusp of the aortic valve. Ablations (30 to 60 s in duration) were performed under fluoroscopic guidance with electrophysiological monitoring. Interrogation of pacemakers and electrocardiography (ECG) determined the persistence of heart block. Ovine hearts were also examined immunohistochemically for localization of conduction tissue.

RESULTS

AVB was produced in eight animals using an atypical approach from the left side of the heart. One animal died due to ventricular fibrillation during ablation proximal to the tricuspid annulus and one lamb was sacrificed postoperatively due to stroke. Four sheep were kept for long-term follow-up (109.8 ± 32.9 d) and required continuous ventricular pacing attributable to lasting AVB, despite significant increases in body weight and size.

CONCLUSIONS

We have created a large animal model of pediatric complete heart block that is stable and technically practicable. We anticipate that this lamb model will allow for advancement of cell-based and other innovative treatments to repair complete heart block in children.

Adenoviral gene transfer of HCN4 creates a genetic pacemaker in pigs with complete atrioventricular block

The hyperpolarization-activated, cyclic nucleotide-gated cation channels (HCN) have been identified as a key factor of cardiac pacemaker activity. The present study investigated the feasibility of using transfection of HCN4, an important subunit in the HCN family, to cure an experimental cardiac bradyarrhythmia. Two adenoviral vectors containing HCN4 and GFP (Ad-HCN4) or GFP alone (Ad-GFP) were constructed. Three or four days after gene injection, the pigs underwent catheter ablation of the atrioventricular (AV) node. After a complete AV block was created, the idioventricular heart rate in the Ad-HCN4 group was significantly greater than in controls. The heart rhythm in the Ad-HCN4 group could be modulated by the beta-adrenergic agonist isoproterenol. An I(f) current could be observed in the ventricular myocytes of the Ad-HCN4 group under patch clamp technique investigations. The expected cell membrane localization of GFP-tagged HCN4 expression was confirmed with confocal fluorescent microscopy. The successful in vivo transfection with Ad-HCN4 was also identified by real-time reverse transcription polymerase chain reaction (RT-PCR). Our study suggested that site-specific gene therapy for cardiac bradyarrhythmias using adenoviral vectors to overexpress HCN4 channels might be feasible.

Biological pacemaking: a concept whose time has come...or is coming

"..when biological pacemakers reach clinical testing it is likely that some form of tandem therapy [with electronic pacemakers] will be used.."

Creating a cardiac pacemaker by gene therapy

While electronic cardiac pacing in its various modalities represents standard of care for treatment of symptomatic bradyarrhythmias and heart failure, it has limitations ranging from absent or rudimentary autonomic modulation to severe complications. This has prompted experimental studies to design and validate a biological pacemaker that could supplement or replace electronic pacemakers. Advances in cardiac gene therapy have resulted in a number of strategies focused on beta-adrenergic receptors as well as specific ion currents that contribute to pacemaker function. This article reviews basic pacemaker physiology, as well as studies in which gene transfer approaches to develop a biological pacemaker have been designed and validated in vivo. Additional requirements and refinements necessary for successful biopacemaker function by gene transfer are discussed.