Next-generation pacemakers: from electrical devices to biological pacemakers

The invention of an electric pacemaker in the middle of the 20th century led to a revolution in the treatment of cardiac conduction system diseases. The improvement of pacemakers continued. In 1962, the first small series of external pacemakers for percutaneous and direct stimulation was produced in Kaunas. After a while, electric pacemakers became more reliable, smaller and lighter in weight, but the problem of foreign body associated infection and limited service life remained unresolved. Modern high-tech medicine strives to create less invasive electric pacemakers, but nevertheless, biological pacemakers can expand the therapeutic arsenal for the treatment of cardiac patients, being the most physiological for humans. The concept of an artificial biological pacemaker consists of the creation of an organic structure that generates a spontaneous rhythm from the implantation site in the myocardium. Various gene and cellular approaches were used to create biological pacemakers: a functional reorganization approach (use of adenovirus vectors for hyperexpression of genes encoding ion channels in cardiomyocytes); hybrid approach (use of fibroblasts to deliver genes of ion channels that provide heart automation); somatic reprogramming approach (overexpression of the transcription factor TBX18 using adenoviral vectors, which reprograms cardiomyocytes into induced sinoatrial node cells, creating cardiac stimulatory activity); cellular approach (transplantation of stem cells to a specific place in the heart, thereby creating biological stimulation). Modern methods of electrical cardiac stimulation and the developed concepts of the biological pacemaker clearly show the possibility of eliminating current problems associated with the use of an artificial pacemaker by replacing it with a biological one. Each of the approaches (gene, cellular, hybrid-cellular, somatic reprogramming) has its own advantages and disadvantages, which predisposes to further study and improvement in order to introduce a biological pacemaker into clinical practice.

Toward Biological Pacing by Cellular Delivery of Hcn2/SkM1

Electronic pacemakers still face major shortcomings that are largely intrinsic to their hardware-based design. Radical improvements can potentially be generated by gene or cell therapy-based biological pacemakers. Our previous work identified adenoviral gene transfer of Hcn2 and SkM1, encoding a “funny current” and skeletal fast sodium current, respectively, as a potent combination to induce short-term biological pacing in dogs with atrioventricular block. To achieve long-term biological pacemaker activity, alternative delivery platforms need to be explored and optimized. The aim of the present study was therefore to investigate the functional delivery of Hcn2/SkM1 via human cardiomyocyte progenitor cells (CPCs). Nucleofection of Hcn2 and SkM1 in CPCs was optimized and gene transfer was determined for Hcn2 and SkM1 in vitro. The modified CPCs were analyzed using patch-clamp for validation and characterization of functional transgene expression. In addition, biophysical properties of Hcn2 and SkM1 were further investigated in lentivirally transduced CPCs by patch-clamp analysis. To compare both modification methods in vivo, CPCs were nucleofected or lentivirally transduced with GFP and injected in the left ventricle of male NOD-SCID mice. After 1 week, hearts were collected and analyzed for GFP expression and cell engraftment. Subsequent functional studies were carried out by computational modeling. Both nucleofection and lentiviral transduction of CPCs resulted in functional gene transfer of Hcn2 and SkM1 channels. However, lentiviral transduction was more efficient than nucleofection-mediated gene transfer and the virally transduced cells survived better in vivo. These data support future use of lentiviral transduction over nucleofection, concerning CPC-based cardiac gene delivery. Detailed patch-clamp studies revealed Hcn2 and Skm1 current kinetics within the range of previously reported values of other cell systems. Finally, computational modeling indicated that CPC-mediated delivery of Hcn2/SkM1 can generate stable pacemaker function in human ventricular myocytes. These modeling studies further illustrated that SkM1 plays an essential role in the final stage of diastolic depolarization, thereby enhancing biological pacemaker functioning delivered by Hcn2. Altogether these studies support further development of CPC-mediated delivery of Hcn2/SkM1 and functional testing in bradycardia models.

Rendimiento de los Marcapasos Biológicos en Modelos Animales de Bloqueo AV: Una Revisión del Alcance

The electronic pacemakers have some complications and limitations for which in the last two decades it has been investigated about biological pacemakers as an alternative treatment. The present study undertakes a scoping review of research on biological pacemakers assessed in an in vivo model of complete heart block to determine which approach has promoted the greatest restauration of heart rate. To achieve that, some databases were used to identify papers published 2011-2021, from which we retrieved 151 papers and after the identification and screening process only 6 articles were included. Of these articles, 4 articles had a pig as an animal model and 2 rats. The most common approach to design a biological pacemaker is gene therapy, alone or with cells in a hybrid approach. Only one study had a cell-based approach, which also achieved the heart rate closest to the normal physiological range of pigs. For rat’s models, the heart rate reported after the complete heart block were not physiologically relevant. In conclusion, the most promising therapy is the one based on cells, because it maintained the heart rate of the animal model within relevant physiological values over 2 weeks. In addition, to develop a permanent biological pacemaker is essential research about a better persistence of the expression for gene-based approach and long-term function assessment for any approach.

Reciprocal interaction between IK1 and If in biological pacemakers: A simulation study

Pacemaking dysfunction (PD) may result in heart rhythm disorders, syncope or even death. Current treatment of PD using implanted electronic pacemaker has some limitations, such as finite battery life and the risk of repeated surgery. As such, the biological pacemaker has been proposed as a potential alternative to the electronic pacemaker for PD treatment. Experimentally it has been shown that bio-engineered pacemaker cells can be generated from non-rhythmic ventricular myocytes (VMs) by knocking down genes related to the inward rectifier potassium channel current (IK1) or by overexpressing hyperpolarization-activated cyclic nucleotide gated channel genes responsible for the “funny” current (If). Such approaches can turn the VM cells into rhythmic pacemaker cells. However, it is unclear if a bio-engineered pacemaker based on the modification of IK1- and If-related channels simultaneously would enhance the ability and stability of bio-engineered pacemaking action potentials (APs). This study aimed to investigate by a computational approach the combined effects of modifying IK1 and If density on the initiation of pacemaking activity in human ventricular cell models. First, the possible mechanism(s) responsible for VMs to generate spontaneous pacemaking APs by changing the density of IK1 and If were investigated. Then the integral action of targeting both IK1 and If simultaneously on the pacemaking APs was analysed. Our results showed a reciprocal interaction between IK1 and If on generating stable and robust pacemaking APs in VMs. In addition, we thoroughly investigated the dynamical behaviours of automatic rhythms in VMs in the IK1 and If parameter space, providing optimal parameter ranges for a robust pacemaker cell. In conclusion, to the best of our knowledge, this study provides a novel theoretical basis for generating stable and robust pacemaker cells from non-pacemaking VMs, which may be helpful in designing engineered biological pacemakers for application purposes.Author SummaryPacemaking dysfunction has become one of the most serious cardiac diseases, which may result in arrhythmia and even death. The treatment of pacemaking dysfunction by electronic pacemaker has saved millions of people in the past fifty years. But not every patient can benefit from it because of possible limitations, such as surgical implication and lack of response to autonomic stimulus. The development of bio-pacemaker based on gene engineering technology provides a promising alternative to electronic pacemaker by manipulating the gene expression of cardiac cells. However, it is still unclear how a stable and robust bio-pacemaker can be generated. The present study aims to elucidate possible mechanisms responsible for a bio-engineered pacemaker by using a computational electrophysiological model of pacemaking cells based on modifying ion channel properties of IK1 and incorporating If in a human ventricular cell model, mimicking experimental approaches of gene engineering. Using the model, possible pacemaking mechanisms in non-pacemaking cells, as well as factors responsible for generating robust and stable biological pacemaker, were investigated. It was shown that the reciprocal interaction between reduction of IK1 and incorporation of If played an important role for producing robust and stable pacemaking. This study provides a novel insight into understanding of the initiation of pacemaking behaviours in non-rhythmic cardiac myocytes, providing a theoretical basis for experimental designing of biological pacemakers.

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 (IK1) 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 (ICaL) plays a major part in the rapid depolarization of the action potential (AP). A small enough ICaL would lead to the failure of the automaticity of the ventricular myocyte. Meanwhile, the background sodium current (IbNa), the background calcium current (IbCa), and the Na+/Ca2+ exchanger current (INaCa) contribute significantly to the slow depolarization, indicating that these currents are the main supplementary power of the pacing induced by depressing IK1, while in the 2D simulation, we find that the weak electrical coupling plays a more important role in the driving of a biological pacemaker.

Genetically Modified Porcine Mesenchymal Stem Cells by Lentiviral Tbx18 Create a Biological Pacemaker

Background. Tbx18 is a vital transcription factor involved in embryonic sinoatrial node (SAN) formation process but is gradually vanished after birth. Myocardial injection of lentiviral Tbx18 converts cardiomyocytes into pacemaker-like cells morphologically and functionally. In this in vitro and in vivo study, genetical modification of porcine bone mesenchymal stem cells (BMSCs) by recapturing the Tbx18 expression creates a biological pacemaker which was examined. Methods. The isolated porcine BMSCs were transfected with lentiviral Tbx18, and the induced pacemaker-like cells were analyzed using real-time polymerase chain reaction and western blotting to investigate the efficiency of transformation. Then, the induced pacemaker-like cells were implanted into the right ventricle of the SAN dysfunction porcine model after the differentiation process. Biological pacemaker activity and ectopic pacing region were tested by an electrocardiograph (ECG) monitor. Results. The isolated porcine BMSCs expressed specific surface markers of stem cells; meanwhile, the expression of myocardial markers was upregulated significantly after lentiviral Tbx18 transfection. The porcine SAN dysfunction model was constructed by electrocoagulation using a surgical electrotome. The results showed that the mean heart beat (HR) of BMSCs-Tbx18 was significantly higher than that of BMSCs-GFP. An ectopic pacing region was affirmed into the right ventricle by ECG after implantation of BMSCs-Tbx18. Conclusion. It was verified that Lenti-Tbx18 is capable of transducing porcine BMSCs into pacemaker-like cells. Genetically modified porcine BMSCs by lentiviral Tbx18 could create a biological pacemaker. However, further researches in large-scale animals are required to rule out unexpected complications prior to application in clinical practice.