Next-generation pacemakers: from small devices to biological pacemakers

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 pacemakers 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 and computationally, it has been shown that bio-engineered pacemaker cells can be generated from non-rhythmic ventricular myocytes (VMs) by knocking out 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). 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. In this study, the possible mechanism(s) responsible for VMs to generate spontaneous pacemaking activity by regulating IK1 and If density were investigated by a computational approach. Our results showed that there was a reciprocal interaction between IK1 and If in ventricular pacemaker model. The effect of IK1 depression on generating ventricular pacemaker was mono-phasic while that of If augmentation was bi-phasic. A moderate increase of If promoted pacemaking activity but excessive increase of If resulted in a slowdown in the pacemaking rate and even an unstable pacemaking state. The dedicated interplay between IK1 and If in generating stable pacemaking and dysrhythmias was evaluated. Finally, a theoretical analysis in the IK1/If parameter space for generating pacemaking action potentials in different states was provided. In conclusion, to the best of our knowledge, this study provides a wide theoretical insight into understandings for generating stable and robust pacemaker cells from non-pacemaking VMs by the interplay of IK1 and If, which may be helpful in designing engineered biological pacemakers for application purposes.

Emerging intraoral biosensors

Biomedical devices that involved continuous and real-time health-care monitoring have drawn much attention in modern medicine, of which skin electronics and implantable devices are widely investigated. Skin electronics are characterized for their non-invasive access to the physiological signals, and implantable devices are superior at the diagnosis and therapy integration. Despite the significant progress achieved, many gaps remain to be explored to provide a more comprehensive overview of human health. As the connecting point of the outer environment and human systems, the oral cavity contains many unique biomarkers that are absent in skin or inner organs, and hence, this could become a promising alternative locus for designing health-care monitoring devices. In this review, we outline the status of the oral cavity during the communication of the environment and human systems and compare the intraoral devices with skin electronics and implantable devices from the biophysical and biochemical aspects. We further summarize the established diagnosis database and technologies that could be adopted to design intraoral biosensors. Finally, the challenges and potential opportunities for intraoral biosensors are discussed. Intraoral biosensors could become an important complement for existing biomedical devices to constitute a more reliable health-care monitoring system.

Sinoatrial node pacemaker cells share dominant biological properties with glutamatergic neurons

Activation of the heart normally begins in the sinoatrial node (SAN). Electrical impulses spontaneously released by SAN pacemaker cells (SANPCs) trigger the contraction of the heart. However, the cellular nature of SANPCs remains controversial. Here, we report that SANPCs exhibit glutamatergic neuron-like properties. By comparing the single-cell transcriptome of SANPCs with that of cells from primary visual cortex in mouse, we found that SANPCs co-clustered with cortical neurons. Tissue and cellular imaging confirmed that SANPCs contained key elements of glutamatergic neurotransmitter system, expressing genes encoding glutamate synthesis pathway (Gls), ionotropic and metabotropic glutamate receptors (Grina, Gria3, Grm1 and Grm5), and glutamate transporters (Slc17a7). SANPCs highly expressed cell markers of glutamatergic neurons (Snap25 and Slc17a7), whereas Gad1, a marker of GABAergic neurons, was negative. Functional studies revealed that inhibition of glutamate receptors or transporters reduced spontaneous pacing frequency of isolated SAN tissues and spontaneous Ca²⁺ transients frequency in single SANPC. Collectively, our work suggests that SANPCs share dominant biological properties with glutamatergic neurons, and the glutamatergic neurotransmitter system may act as an intrinsic regulation module of heart rhythm, which provides a potential intervention target for pacemaker cell-associated arrhythmias.

Recent advances in bioelectronics chemistry

Research in bioelectronics is highly interdisciplinary, with many new developments being based on techniques from across the physical and life sciences. Advances in our understanding of the fundamental chemistry underlying the materials used in bioelectronic applications have been a crucial component of many recent discoveries. In this review, we highlight ways in which a chemistry-oriented perspective may facilitate novel and deep insights into both the fundamental scientific understanding and the design of materials, which can in turn tune the functionality and biocompatibility of bioelectronic devices. We provide an in-depth examination of several developments in the field, organized by the chemical properties of the materials. We conclude by surveying how some of the latest major topics of chemical research may be further integrated with bioelectronics.

Recent advances in bioelectronics chemistry

Research in bioelectronics is highly interdisciplinary, with many new developments being based on techniques from across the physical and life sciences. Advances in our understanding of the fundamental chemistry underlying the materials used in bioelectronic applications have been a crucial component of many recent discoveries. In this review, we highlight ways in which a chemistry-oriented perspective may facilitate novel and deep insights into both the fundamental scientific understanding and the design of materials, which can in turn tune the functionality and biocompatibility of bioelectronic devices. We provide an in-depth examination of several developments in the field, organized by the chemical properties of the materials. We conclude by surveying how some of the latest major topics of chemical research may be further integrated with bioelectronics.

Heart regeneration: beyond new muscle and vessels

The most striking consequence of a heart attack is the loss of billions of heart muscle cells, alongside damage to the associated vasculature. The lost cardiovascular tissue is replaced by scar formation, which is non-functional and results in pathological remodelling of the heart and ultimately heart failure. It is, therefore, unsurprising that the heart regeneration field has centred efforts to generate new muscle and blood vessels through targeting cardiomyocyte proliferation and angiogenesis following injury. However, combined insights from embryological studies and regenerative models, alongside the adoption of -omics technology, highlight the extensive heterogeneity of cell types within the forming or re-forming heart and the significant crosstalk arising from non-muscle and non-vessel cells. In this review, we focus on the roles of fibroblasts, immune, conduction system, and nervous system cell populations during heart development and we consider the latest evidence supporting a function for these diverse lineages in contributing to regeneration following heart injury. We suggest that the emerging picture of neurologically, immunologically, and electrically coupled cell function calls for a wider-ranging combinatorial approach to heart regeneration.

Ultrasound-Induced Wireless Energy Harvesting: From Materials Strategies to Functional Applications

Wireless energy harvesting represents an emerging technology that can be integrated into a variety of systems for biomedical, physical, and chemical functions. The miniaturization and ease of implementation are the main challenges for the development of wireless energy harvesting systems. Unlike most reported wireless energy harvesting technologies represented by electromagnetic coupling, the new generation of ultrasound-induced wireless energy harvesting (UWEH) that use propagating ultrasound waves to carry the available energy provides a strategy with higher resolution, deeper penetration, and more security, especially in nanodevices and implantable medical systems where a long-term stable power is required. Recently, advances in nanotechnologies, microelectronics, and biomedical systems are revolutionizing UWEH. In this article, an overview of recent developments in UWEH technologies that use a variety of material strategies and system designs based on the piezoelectric and capacitive energy harvesting mechanisms is provided. Practical applications are also presented, including wireless power for bio-implantable devices, direct cell/tissue electrical stimulations, wireless recording and communication in nervous systems, ultrasonic modulated drug delivery, self-powered acoustic sensors, and ultrasound-induced piezoelectric catalysis. Finally, perspectives and opportunities are also highlighted.

NODAL inhibition promotes differentiation of pacemaker-like cardiomyocytes from human induced pluripotent stem cells

Directed cardiomyogenesis from human induced pluripotent stem cells (hiPSCs) has been greatly improved in the last decade but directed differentiation to pacemaking cardiomyocytes (CMs) remains incompletely understood. In this study, we demonstrated that inhibition of NODAL signaling by a specific NODAL inhibitor (SB431542) in the cardiac mesoderm differentiation stage downregulated PITX2c, a transcription factor that is known to inhibit the formation of the sinoatrial node in the left atrium during cardiac development. The resulting hiPSC-CMs were smaller in cell size, expressed higher pro-pacemaking transcription factors, TBX3 and TBX18, and exhibited pacemaking-like electrophysiological characteristics compared to control hiPSC-CMs differentiated from established Wnt-based protocol. The pacemaker-like subtype increased up to 2.4-fold in hiPSC-CMs differentiated with the addition of SB431542 relative to the control. Hence, Nodal inhibition in the cardiac mesoderm stage promoted pacemaker-like CM differentiation from hiPSCs. Improving the yield of human pacemaker-like CMs is a critical first step in the development of functional human cell-based biopacemakers.

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.

A Review of Bioresorbable Implantable Medical Devices: Materials, Fabrication, and Implementation

Implantable medical devices (IMDs) are designed to sense specific parameters or stimulate organs and have been actively used for treatment and diagnosis of various diseases. IMDs are used for long-term disease screening or treatments and cannot be considered for short-term applications since patients need to go through a surgery for retrieval of the IMD. Advances in bioresorbable materials has led to the development of transient IMDs that can be resorbed by bodily fluids and disappear after a certain period. These devices are designed to be implanted in the adjacent of the targeted tissue for predetermined times with the aim of measurement of pressure, strain, or temperature, while the bioelectronic devices stimulate certain tissues. They enable opportunities for monitoring and treatment of acute diseases. To realize such transient and miniaturized devices, researchers utilize a variety of materials, novel fabrication methods, and device design strategies. This review discusses potential bioresorbable materials for each component in an IMD followed by programmable degradation and safety standards. Then, common fabrication methods for bioresorbable materials are introduced, along with challenges. The final section provides representative examples of bioresorbable IMDs for various applications with an emphasis on materials, device functionality, and fabrication methods.

A Chip Integrity Monitor for Evaluating Moisture/Ion Ingress in mm-Sized Single-Chip Implants

For mm-sized implants incorporating silicon integrated circuits, ensuring lifetime operation of the chip within the corrosive environment of the body still remains a critical challenge. For the chip's packaging, various polymeric and thin ceramic coatings have been reported, demonstrating high biocompatibility and barrier properties. Yet, for the evaluation of the packaging and lifetime prediction, the conventional helium leak test method can no longer be applied due to the mm-size of such implants. Alternatively, accelerated soak studies are typically used instead. For such studies, early detection of moisture/ion ingress using an in-situ platform may result in a better prediction of lifetime functionality. In this work, we have developed such a platform on a CMOS chip. Ingress of moisture/ions would result in changes in the resistance of the interlayer dielectrics (ILD) used within the chip and can be tracked using the proposed system, which consists of a sensing array and an on-chip measurement engine. The measurement system uses a novel charge/discharge based time-mode resistance sensor that can be implemented using simple yet highly robust circuitry. The sensor array is implemented together with the measurement engine in a standard 0.18  μm 6-metal CMOS process. The platform was validated through a series of dry and wet measurements. The system can measure the ILD resistance with values of up to 0.504 peta-ohms, with controllable measurement steps that can be as low as 0.8 M Ω. The system works with a supply voltage of 1.8 V, and consumes 4.78 mA. Wet measurements in saline demonstrated the sensitivity of the platform in detecting moisture/ion ingress. Such a platform could be used both in accelerated soak studies and during the implant's life-time for monitoring the integrity of the chip's packaging.

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.

A study of the outward background current conductance gK1, the pacemaker current conductance gf, and the gap junction conductance gj as determinants of biological pacing in single cells and in a two-cell syncytium using the dynamic clamp

We previously demonstrated that a two-cell syncytium, composed of a ventricular myocyte and an mHCN2 expressing cell, recapitulated most properties of in vivo biological pacing induced by mHCN2-transfected hMSCs in the canine ventricle. Here, we use the two-cell syncytium, employing dynamic clamp, to study the roles of g_f (pacemaker conductance), g_K1 (background K⁺ conductance), and g_j (intercellular coupling conductance) in biological pacing. We studied g_f and g_K1 in single HEK293 cells expressing cardiac sodium current channel Na_v1.5 (SCN5A). At fixed g_f, increasing g_K1 hyperpolarized the cell and initiated pacing. As g_K1 increased, rate increased, then decreased, finally ceasing at membrane potentials near E_K. At fixed g_K1, increasing g_f depolarized the cell and initiated pacing. With increasing g_f, rate increased reaching a plateau, then decreased, ceasing at a depolarized membrane potential. We studied g_j via virtual coupling with two non-adjacent cells, a driver (HEK293 cell) in which g_K1 and g_f were injected without SCN5A and a follower (HEK293 cell), expressing SCN5A. At the chosen values of g_K1 and g_f oscillations initiated in the driver, when g_j was increased synchronized pacing began, which then decreased by about 35% as g_j approached 20 nS. Virtual uncoupling yielded similar insights into g_j. We also studied subthreshold oscillations in physically and virtually coupled cells. When coupling was insufficient to induce pacing, passive spread of the oscillations occurred in the follower. These results show a non-monotonic relationship between g_K1, g_f, g_j, and pacing. Further, oscillations can be generated by g_K1 and g_f in the absence of SCN5A.

Canonical Wnt Signaling Sets the Pace

In this issue of Developmental Cell, Ren et al. (2019) identify the embryonic origin of cardiac pacemaker cells in zebrafish and implicate Wnt5b in promoting their differentiation. Furthermore, canonical Wnt activation in human stem cell-derived cardiac progenitors produces functional pacemaker cells in vitro, advancing the therapeutic potential of biological pacemakers.

Na/Ca exchange in the atrium: Role in sinoatrial node pacemaking and excitation-contraction coupling

Na/Ca exchange is the dominant calcium (Ca) efflux mechanism in cardiac myocytes. Although our knowledge of exchanger function (NCX1 in the heart) was originally established using biochemical and electrophysiological tools such as cardiac sarcolemmal vesicles and the giant patch technique [1-4], many advances in our understanding of the physiological/pathophysiological roles of NCX1 in the heart have been obtained using a suite of genetically modified mice. Early mouse studies focused on modification of expression levels of NCX1 in the ventricles, with transgenic overexpressors, global NCX1 knockout (KO) mice (which were embryonic lethal if homozygous), and finally ventricular-specific NCX1 KO [5-12]. We found, to our surprise, that ventricular cardiomyocytes lacking NCX1 can survive and function by engaging a clever set of adaptations to minimize Ca entry, while maintaining contractile function through an increase in excitation-contraction (EC) coupling gain [5,6,13]. Having studied ventricular NCX1 ablation in detail, we more recently focused on elucidating the role of NCX1 in the atria through altering NCX1 expression. Using a novel atrial-specific NCX1 KO mouse, we found unexpected changes in atrial cell morphology and calcium handling, together with dramatic alterations in the function of sinoatrial node (SAN) pacemaker activity. In this review, we will discuss these findings and their implications for cardiac disease.

Influence and safety of electronic apex locators in patients with cardiovascular implantable electronic devices: a systematic review

The widespread use of cardiovascular implantable electronic devices has increased concerns regarding using electronic apex locators in patients with these devices. This systematic review investigated the effects and safety of using electronic apex locators in patients with cardiovascular implantable electronic devices.

METHODS

An electronic search in the Cochrane Library, PubMed (MEDLINE), ScienceDirect, and Scientific Electronic Library Online (Scielo) databases for relevant articles published between December 2000 and December 2018 was performed. The search strategy centered on terms related to electronic apex locators use during root canal treatment in patients with cardiovascular implantable electronic devices.

RESULTS

Seven studies (five in vitro and two in vivo) fulfilled the inclusion criteria for this review. It was found that electronic apex locators can be used safely in patients with cardiovascular implantable electronic devices, when general precautions are followed.

CONCLUSIONS

Although the present review suggests that electronic apex locators can be used safely in patients with implantable cardioverter defibrillators, consultation with patients' cardiologists remains advisable.

Biological Pacemakers: Still a Dream?

A biological pacemaker is one or more types of cellular components that, when implanted into certain regions of the heart, produce electrical stimuli that mimic that of the body's natural pacemaker cells. Somatic gene transfer, cell fusion, or cell transplantation provide a way to realize it as 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 possibilities. The idea, no doubt, is bright and appealing. The objective herein intends to dig into the subject trying to find out how realizable it really is.

Transcription Factor prrx1 Promotes Brown Adipose-Derived Stem Cells Differentiation to Sinus Node-Like Cells

This study investigated whether overexpression of paired-related homeobox 1 (prrx1) can successfully induce differentiation of brown adipose-derived stem cells (BADSCs) into sinus node-like cells. The experiments were performed in two groups: adenovirus-green fluorescent protein (Ad-GFP) group and Ad-prrx1 group. After 5-7 days of adenoviral transfection, the expression levels of sinus node cell-associated pacing protein (hyperpolarization-activated cyclic nucleotide-gated potassium channel 4 [HCN4]) and ion channel (calcium channel, voltage-dependent, T type, alpha 1G subunit [Cacna1g]), as well as transcription factors (T-box 18 [TBX18], insulin gene enhancer binding protein 1 [ISL-1], paired-like homeodomain transcription factor 2 [pitx2], short stature homeobox 2 [shox2]), were detected by western blot and reverse transcription-quantitative polymerase chain reaction. Immunofluorescence assay was carried out to detect whether prrx1 was coexpressed with HCN4, TBX18, and ISL-1. Finally, whole-cell patch-clamp technique was used to record pacing current hyperpolarization-activated inward current (I_f). The isolated cells were CD90⁺, CD29⁺, and CD45^-, indicating that pure BADSCs were successfully isolated. After 5-7 days of Ad transfection into cells, the mRNA levels and protein levels of pacing-related factors (TBX18, ISL-1, HCN4, shox2, and Cacna1g) in Ad-prrx1 group were significantly higher than those in Ad-GFP group. However, the expression level of pitx2 was decreased. Immunofluorescence analysis showed that prrx1 was coexpressed with TBX18, ISL-1, and HCN4 in the Ad-prrx1 group, which did not appear in the Ad-GFP group. Whole-cell patch clamps were able to record the I_f current in the experimental group rather than in the Ad-GFP group. Overexpression of prrx1 can successfully induce sinus node-like cells.

Electrically Insulating Plasma Polymer/ZnO Composite Films

In this report, the electrical properties of plasma polymer films functionalized with ZnO nanoparticles were investigated with respect to their potential applications in biomaterials and microelectronics fields. The nanocomposite films were produced using a single-step method that combines simultaneous plasma polymerization of renewable geranium essential oil with thermal decomposition of zinc acetylacetonate Zn(acac)2. The input power used for the deposition of composites were 10 W and 50 W, and the resulting composite structures were abbreviated as Zn/Ge 10 W and Zn/Ge 50 W, respectively. The electrical properties of pristine polymers and Zn/polymer composite films were studied in metal-insulator-metal structures. At a quantity of ZnO of around ~1%, it was found that ZnO had a small influence on the capacitance and dielectric constants of thus-fabricated films. The dielectric constant of films with smaller-sized nanoparticles exhibited the highest value, whereas, with the increase in ZnO particle size, the dielectric constant decreases. The conductivity of the composites was calculated to be in the in the range of 10-14-10-15 Ω-1 m-1, significantly greater than that for the pristine polymer, the latter estimated to be in the range of 10-16-10-17 Ω-1 m-1.

Transcriptional regulation of the cardiac conduction system

The rate and rhythm of heart muscle contractions are coordinated by the cardiac conduction system (CCS), a generic term for a collection of different specialized muscular tissues within the heart. The CCS components initiate the electrical impulse at the sinoatrial node, propagate it from atria to ventricles via the atrioventricular node and bundle branches, and distribute it to the ventricular muscle mass via the Purkinje fibre network. The CCS thereby controls the rate and rhythm of alternating contractions of the atria and ventricles. CCS function is well conserved across vertebrates from fish to mammals, although particular specialized aspects of CCS function are found only in endotherms (mammals and birds). The development and homeostasis of the CCS involves transcriptional and regulatory networks that act in an embryonic-stage-dependent, tissue-dependent, and dose-dependent manner. This Review describes emerging data from animal studies, stem cell models, and genome-wide association studies that have provided novel insights into the transcriptional networks underlying CCS formation and function. How these insights can be applied to develop disease models and therapies is also discussed.

4D cardiac electromechanical activation imaging

Cardiac abnormalities, a major cause of morbidity and mortality, affect millions of people worldwide. Despite the urgent clinical need for early diagnosis, there is currently no noninvasive technique that can infer to the electrical function of the whole heart in 3D and thereby localize abnormalities at the point of care. Here we present a new method for noninvasive 4D mapping of the cardiac electromechanical activity in a single heartbeat for heart disease characterization such as arrhythmia and infarction. Our novel technique captures the 3D activation wave of the heart in vivo using high volume-rate (500 volumes per second) ultrasound with a 32 × 32 matrix array. Electromechanical activation maps are first presented in a normal and infarcted cardiac model in silico and in canine heart during pacing and re-entrant ventricular tachycardia in vivo. Noninvasive 4D electromechanical activation mapping in a healthy volunteer and a heart failure patient are also determined. The technique described herein allows for direct, simultaneous and noninvasive visualization of electromechanical activation in 3D, which provides complementary information on myocardial viability and/or abnormality to clinical imaging.

Evaluating and managing bradycardia

Bradycardia is a commonly observed arrhythmia and a frequent occasion for cardiac consultation. Defined as a heart rate of less than 50-60 bpm, bradycardia can be observed as a normal phenomenon in young athletic individuals, and in patients as part of normal aging or disease (Table 1). Pathology that produces bradycardia may occur within the sinus node, atrioventricular (AV) nodal tissue, and the specialized His-Purkinje conduction system. Given the overlap of heart rate ranges with non-pathologic changes, assessment of symptoms is a critical component in the evaluation and management of bradycardia. Treatment should rarely be prescribed solely on the basis of a heart rate lower than an arbitrary cutoff or a pause above certain duration. In the 2018 ACC/AHA/HRS Guideline on the Evaluation and Management of Patients with Bradycardia and Cardiac Conduction Delay (referred to hereafter as the 2018 Bradycardia Guideline), there was a significant shift in emphasis from prior guidelines that emphasized device-based implantation recommendations to a focus on evaluation and management of disease states [1,2]. In this review, we will highlight the changes in the new guideline as well as describe the key elements in evaluation and management of patients presenting with bradycardia.

Pacemaker cell characteristics of differentiated and HCN4-transduced human mesenchymal stem cells

AIMS

Cell-based biological pacemakers aim to overcome limitations and side effects of electronic pacemaker devices. We here developed and tested different approaches to achieve nodal-type differentiation using human adipose- and bone marrow-derived mesenchymal stem cells (haMSC, hbMSC).

MAIN METHODS

haMSC and hbMSC were differentiated using customized protocols. Quantitative RT-PCR was applied for transcriptional pacemaker-gene profiling. Protein membrane expression was analyzed by immunocytochemistry. Pacemaker current (I_f) was studied in haMSC with and without lentiviral HCN4-transduction using patch clamp recordings. Functional characteristics were evaluated by co-culturing with neonatal rat ventricular myocytes (NRVM).

KEY FINDINGS

Culture media-based differentiation for two weeks generated cells with abundant transcription of ion channel genes (Ca_v1.2, NCX1), transcription factors (TBX3, TBX18, SHOX2) and connexins (Cx31.9 and Cx45) characteristic for cardiac pacemaker tissue, but lack adequate HCN transcription. haMSC-derived cells revealed transcript levels, which were closer related to sinoatrial nodal cells than hbMSC-derived cells. To substitute for the lack of I_f, we performed lentiviral HCN4-transduction of haMSC resulting in stable I_f. Co-culturing with NRVM demonstrated that differentiated haMSC expressing HCN4 showed earlier onset of spontaneous contractions and higher beating regularity, synchrony and rate compared to co-cultures with non-HCN4-transduced haMSC or HCN4-transduced, non-differentiated haMSC. Confocal imaging indicated increased membrane expression of cardiac gap junctional proteins in differentiated haMSC.

SIGNIFICANCE

By differentiation haMSC, rather than hbMSC attain properties favorable for cardiac pacemaking. In combination with lentiviral HCN4-transduction, a cellular phenotype was generated that sustainably controls and stabilizes rate in co-culture with NRVM.

Congenital heart block: Pace earlier (Childhood) than later (Adulthood)

Congenital complete heart block (CCHB) occurs in 2-5% of pregnancies with positive anti-Ro/SSA and/or anti-La/SSB antibodies, and has a recurrence rate of 12-25% in a subsequent pregnancy. After trans-placental passage, these autoantibodies attack and destroy the atrioventricular (AV) node in susceptible fetuses with the highest-risk period observed between 16 and 28 weeks' gestational age. Many mothers are asymptomatic carriers, while <1/3 have a preexisting diagnosis of a rheumatic disease. The mortality of CCHB is predominant in utero and in the first months of life, reaching 15-30%. The diagnosis of CCHB can be confirmed by fetal echocardiography before birth and by electrocardiography after birth. Whether early in-utero detection and treatment might prevent or reverse this condition remains controversial. In addition to autoantibody-associated CCHB, there is also an isolated (absent structural heart disease) nonimmune early- or late-onset heart block detected later in childhood that may be associated with specific genetic markers or other pathogenic mechanisms. In isolated immune or non-immune CCHB, cardiac pacemakers are implanted in symptomatic patients, however, data on the natural history of CCHB in the adult life indicate that all patients, even if asymptomatic, should receive a pacemaker when first diagnosed. However, important issues have emerged in these patients wherein life-long conventional right ventricular apical pacing may produce left ventricular dysfunction (pacing-induced cardiomyopathy) necessitating a priori alternate site pacing or subsequent upgrading to biventricular pacing. All these issues are herein reviewed and two algorithms are proposed for diagnosis and management of CCHB in the fetus and in the older individual.

Bioresorbable Electronic Implants: History, Materials, Fabrication, Devices, and Clinical Applications

Medical implants, either passive implants for structural support or implantable devices with active electronics, have been widely used for the diagnosis and treatment of various diseases and clinical issues. These implants offer various functions, including mechanical support of biological structures in orthopedic and dental applications, continuous electrophysiological monitoring and feedback of electrical stimulation in neuronal and cardiac applications, and controlled drug delivery while maintaining arterial structure in drug-eluting stents. Although these implants exhibit long-term biocompatibility, surgery for their retrieval is often required, which imposes physical, biological, and economical burdens on the patients. Therefore, as an alternative to such secondary surgeries, bioresorbable implants that disappear after a certain period of time inside the body, including bioresorbable active electronics, have been highlighted recently. This review first discusses the historical background of medical implants and briefly define related terminology. Representative examples of non-degradable medical implants for passive structural support and/or for diagnosis and therapy with active electronics are also provided. Then, recent progress in bioresorbable active implants composed of biosignal sensors, actuators for therapeutics, wireless power supply components, and their integrated systems are reviewed. Finally, clinical applications of these bioresorbable electronic implants are exemplified with brief conclusion and future outlook.

A rat model of complete atrioventricular block recapitulates clinical indices of bradycardia and provides a platform to test disease-modifying therapies

Complete atrioventricular block (CAVB) is a life-threatening arrhythmia. A small animal model of chronic CAVB that properly reflects clinical indices of bradycardia would accelerate the understanding of disease progression and pathophysiology, and the development of therapeutic strategies. We sought to develop a surgical model of CAVB in adult rats, which could recapitulate structural remodeling and arrhythmogenicity expected in chronic CAVB. Upon right thoracotomy, we delivered electrosurgical energy subepicardially via a thin needle into the atrioventricular node (AVN) region of adult rats to create complete AV block. The chronic CAVB animals developed dilated and hypertrophied ventricles with preserved systolic functions due to compensatory hemodynamic remodeling. Ventricular tachyarrhythmias, which are difficult to induce in the healthy rodent heart, could be induced upon programmed electrical stimulation in chronic CAVB rats and worsened when combined with β-adrenergic stimulation. Focal somatic gene transfer of TBX18 to the left ventricular apex in the CAVB rats resulted in ectopic ventricular beats within days, achieving a de novo ventricular rate faster than the slow atrioventricular (AV) junctional escape rhythm observed in control CAVB animals. The model offers new opportunities to test therapeutic approaches to treat chronic and severe CAVB which have previously only been testable in large animal models.

Transcriptome analysis of mouse and human sinoatrial node cells reveals a conserved genetic program

The rate of contraction of the heart relies on proper development and function of the sinoatrial node, which consists of a small heterogeneous cell population, including Tbx3⁺ pacemaker cells. Here, we have isolated and characterized the Tbx3⁺ cells from Tbx3 ^+/Venus knock-in mice. We studied electrophysiological parameters during development and found that Venus-labeled cells are genuine Tbx3⁺ pacemaker cells. We analyzed the transcriptomes of late fetal FACS-purified Tbx3⁺ sinoatrial nodal cells and Nppb-Katushka⁺ atrial and ventricular chamber cardiomyocytes, and identified a sinoatrial node-enriched gene program, including key nodal transcription factors, BMP signaling and Smoc2, the disruption of which in mice did not affect heart rhythm. We also obtained the transcriptomes of the sinoatrial node region, including pacemaker and other cell types, and right atrium of human fetuses, and found a gene program including TBX3, SHOX2, ISL1 and HOX family members, and BMP and NOTCH signaling components conserved between human and mouse. We conclude that a conserved gene program characterizes the sinoatrial node region and that the Tbx3 ^+/Venus allele provides a reliable tool for visualizing the sinoatrial node, and studying its development and function.

A new combination of transcription factors increases the harvesting efficiency of pacemaker‑like cells

Biological pacemakers that combine cell-based and gene-based therapies are a promising treatment for sick sinus syndrome or severe atrioventricular block. The current study aimed to induce differentiation of adipose tissue-derived stem cells (ADSCs) into cardiac pacemaker cells through co-expression of the transcription factors insulin gene enhancer binding protein 1 (ISL-1) and T-box18 (Tbx18). ADSCs were transfected with green fluorescent protein, ISL-1, Tbx18 or ISL-1+Tbx18 fluorescent protein lentiviral vectors, and subsequently co-cultured with neonatal rat ventricular cardiomyocytes in vitro for 7 days. The potential for regulating the differentiation of ADSCs into pacemaker-like cells was evaluated by cell morphology, beating rate, reverse transcription-quantitative polymerase chain reaction, western blotting, immunofluorescence and electrophysiological activity. ADSCs were successfully transformed into spontaneously beating cells that exhibited a behavior similar to that of co-cultured pacemaker cells. This effect was significantly increased in the combined ISL-1 and Tbx18 group. These results provide a potential strategy for enriching the cardiac pacemaker cell population from ADSCs.

Implantation of engineered conduction tissue in the rat heart

Engineered conduction tissues (ECTs) are cardiac conduction tissues fabricated in vitro to allow for more precisely targeted in vivo transplantation therapy. The transplantation of ECTs may be ideal for the treatment of atrioventricular conduction block and could have a significant impact on the future application of biological pacemakers. However, there is little published information regarding the conduction function of ECTs in vivo. In the present study, ECTs were constructed by seeding cardiac progenitor cells (CPCs) into a collagen sponge and were then transplanted into animal hearts to determine whether they could act as an atrioventricular conduction pathway. The results demonstrated that the transplanted ECTs were adequately vascularized at the early stage of transplantation and could survive in the atrioventricular junction area of rats. A large number of myocardial tissue (≥29% of the new muscle fiber tissue formation area in the implanted ECTs) were observed by Masson's trichrome staining at 60 days post-transplantation. Positive staining for connexin-40, connexin-43, HCN2 and cTnT was exhibited during the period of 20 to 90 days post-transplantation. This result suggested that the transplanted ECTs formed gap junctions with the allogeneic myocardium and developed into cardiac conduction tissues with certain myocardial components. Electrocardiography (ECG) confirmed that there was a clear pre-excitation syndrome in the rats transplanted with ECTs during the period of 20 to 90 days post-transplantation. The recovery rate in the rats implanted with ECTs was 61.54% within 1 h following atrioventricular block, and the heart rhythm following recovery was close to normal. By contrast, the recovery rate was only 4.17% in the rats implanted with blank collagen sponges (BCSs), and none of the sham rats exhibited atrioventricular block recovery. In conclusion, ECTs can survive and mechanically integrate with the allogeneic myocardium following transplantation into rat hearts. An atrioventricular accessory pathway similar to Kent bundles could be established between the atria and ventricles of rats following implantation. It is suggested that ECTs may be a potential substitution therapy for atrioventricular conduction block.

Functional biological pacemaker generation by T-Box18 protein expression via stem cell and viral delivery approaches in a murine model of complete heart block

Despite recent advances in the treatment of cardiac arrhythmia, the available options are still limited and associated with some complications. Induction of biological pacemakers via Tbx18 gene insertion in the heart tissue has been suggested as a promising therapeutic strategy for cardiac arrhythmia. Following a previous in vitro study reporting the production of Tbx18-expressing human induced pluripotent stem cell-derived cardiomyocytes (hiPS-CMs), we aimed to investigate the efficacy of these engineered cells to generate pacemaker rhythms in a murine model of complete heart block. We also attempted to generate a functional pacemaker by Tbx18 overexpression in native cardiac cells of rat heart. The hiPSC-derived pacemaker cells were produced by lentiviral delivery of Tbx18 gene to stem cells during a small molecule-based differentiation process. In the present study, 16 male albino Wistar rats were randomly assigned to Tbx18-lentivirus (n = 4) and Tbx18-pacemaker cells (n = 4) administered via injection into the left ventricular anterolateral wall. The control rats received GFP-lentiviruses (n = 4) and GFP-pacemaker cells (n = 4). Fourteen days after the injection, the rats were sacrificed and analyzed by electrocardiography (ECG) recording using a Langendorff-perfused heart model following complete heart block induced by hypokalemia and crashing. Immunofluorescence staining was used to investigate the expression of Tbx18, HCN4 and connexin 43 (Cx43) proteins in Tbx18-delivered cells of heart tissues. The heart rate was significantly reduced after complete heart block in all of the experimental rats (P < 0.05). Heart beating in the Tbx18-transduced hearts was slower compared with rats receiving Tbx18-pacemaker cells (P = 0.04). The duration of ventricular fibrillation (VF) was higher in the lentiviral Tbx18 group compared with the GFP-injected controls (P = 0.02) and the Tbx18-pacemaker cell group (P = 0.02). The ECG recording data showed spontaneous pacemaker rhythms in both intervention groups with signal propagation in Tbx18-transduced ventricles. Immunostaining results confirmed the overexpression of HCN4 and downregulation of Cx43 as a result of the expression of the Tbx18 gene and spontaneously contracting myocyte formation. We confirmed the formation of a functional pacemaker after introduction of Tbx18 via cell and gene therapy strategies. Although the pacemaker activity was better in gene-received hearts since there were longer VF duration and signal propagation from the injection site, more data should be gathered from the long-term activity of such pacemakers in different hosts.

Cardiomyocyte Progenitor Cells as a Functional Gene Delivery Vehicle for Long-Term Biological Pacing

Sustained pacemaker function is a challenge in biological pacemaker engineering. Human cardiomyocyte progenitor cells (CMPCs) have exhibited extended survival in the heart after transplantation. We studied whether lentivirally transduced CMPCs that express the pacemaker current I_f (encoded by HCN4) can be used as functional gene delivery vehicle in biological pacing. Human CMPCs were isolated from fetal hearts using magnetic beads coated with Sca-1 antibody, cultured in nondifferentiating conditions, and transduced with a green fluorescent protein (GFP)- or HCN4-GFP-expressing lentivirus. A patch-clamp analysis showed a large hyperpolarization-activated, time-dependent inward current (−20 pA/pF at −140 mV, n = 14) with properties typical of I_f in HCN4-GFP-expressing CMPCs. Gap-junctional coupling between CMPCs and neonatal rat ventricular myocytes (NRVMs) was demonstrated by efficient dye transfer and changes in spontaneous beating activity. In organ explant cultures, the number of preparations showing spontaneous beating activity increased from 6.3% in CMPC/GFP-injected preparations to 68.2% in CMPC/HCN4-GFP-injected preparations (P < 0.05). Furthermore, in CMPC/HCN4-GFP-injected preparations, isoproterenol induced a significant reduction in cycle lengths from 648 ± 169 to 392 ± 71 ms (P < 0.05). In sum, CMPCs expressing HCN4-GFP functionally couple to NRVMs and induce physiologically controlled pacemaker activity and may therefore provide an attractive delivery platform for sustained pacemaker function.

Optical stimulation of cardiac cells with a polymer-supported silicon nanowire matrix

Electronic pacemakers can treat electrical conduction disorders in hearts; however, they are invasive, bulky, and linked to increased incidence of infection at the tissue-device interface. Thus, researchers have looked to other more biocompatible methods for cardiac pacing or resynchronization, such as femtosecond infrared light pulsing, optogenetics, and polymer-based cardiac patches integrated with metal electrodes. Here we develop a biocompatible nongenetic approach for the optical modulation of cardiac cells and tissues. We demonstrate that a polymer-silicon nanowire composite mesh can be used to convert fast moving, low-radiance optical inputs into stimulatory signals in target cardiac cells. Our method allows for the stimulation of the cultured cardiomyocytes or ex vivo heart to beat at a higher target frequency.

Insulin gene enhancer binding protein 1 induces adipose tissue‑derived stem cells to differentiate into pacemaker‑like cells

Hybrid approaches combining gene- and cell-based therapies to make biological pacemakers are a promising therapeutic avenue for bradyarrhythmia. The present study aimed to direct adipose tissue-derived stem cells (ADSCs) to differentiate specifically into cardiac pacemaker cells by overexpressing a single transcription factor, insulin gene enhancer binding protein 1 (ISL-1). In the present study, the ADSCs were transfected with ISL‑1 or mCherry fluorescent protein lentiviral vectors and co-cultured with neonatal rat ventricular cardiomyocytes (NRVMs) in vitro for 5-7 days. The feasibility of regulating the differentiation of ADSCs into pacemaker-like cells by overexpressing ISL-1 was evaluated by observation of cell morphology and beating rate, reverse transcription-quantitative polymerase chain reaction analysis, western blotting, immunofluorescence and analysis of electrophysiological activity. In conclusion, these data indicated that the overexpression of ISL-1 in ADSCs may enhance the pacemaker phenotype and automaticity in vitro, features which were significantly increased following co‑culture induction.

Gene Therapy Approaches to Biological Pacemakers

Bradycardia arising from pacemaker dysfunction can be debilitating and life threatening. Electronic pacemakers serve as effective treatment options for pacemaker dysfunction. They however present their own limitations and complications. This has motivated research into discovering more effective and innovative ways to treat pacemaker dysfunction. Gene therapy is being explored for its potential to treat various cardiac conditions including cardiac arrhythmias. Gene transfer vectors with increasing transduction efficiency and biosafety have been developed and trialed for cardiovascular disease treatment. With an improved understanding of the molecular mechanisms driving pacemaker development, several gene therapy targets have been identified to generate the phenotypic changes required to correct pacemaker dysfunction. This review will discuss the gene therapy vectors in use today along with methods for their delivery. Furthermore, it will evaluate several gene therapy strategies attempting to restore biological pacing, having the potential to emerge as viable therapies for pacemaker dysfunction.

Embryonic Tbx3+ cardiomyocytes form the mature cardiac conduction system by progressive fate restriction

A small network of spontaneously active Tbx3+ cardiomyocytes forms the cardiac conduction system (CCS) in adults. Understanding the origin and mechanism of development of the CCS network are important steps towards disease modeling and the development of biological pacemakers to treat arrhythmias. We found that Tbx3 expression in the embryonic mouse heart is associated with automaticity. Genetic inducible fate mapping revealed that Tbx3+ cells in the early heart tube are fated to form the definitive CCS components, except the Purkinje fiber network. At mid-fetal stages, contribution of Tbx3+ cells was restricted to the definitive CCS. We identified a Tbx3+ population in the outflow tract of the early heart tube that formed the atrioventricular bundle. Whereas Tbx3+ cardiomyocytes also contributed to the adjacent Gja5+ atrial and ventricular chamber myocardium, embryonic Gja5+ chamber cardiomyocytes did not contribute to the Tbx3+ sinus node or to atrioventricular ring bundles. In conclusion, the CCS is established by progressive fate restriction of a Tbx3+ cell population in the early developing heart, which implicates Tbx3 as a useful tool for developing strategies to study and treat CCS diseases.