Excitation and impulse conduction in the human fetal heart

Modeling the atrioventricular conduction axis using human pluripotent stem cell-derived cardiac assembloids

The atrioventricular (AV) conduction axis provides electrical continuity between the atrial and ventricular chambers. The "nodal" cardiomyocytes populating this region (AV canal in the embryo, AV node from fetal stages onward) propagate impulses slowly, ensuring sequential contraction of the chambers. Dysfunction of AV nodal tissue causes severe disturbances in rhythm and contraction, and human models that capture its salient features are limited. Here, we report an approach for the reproducible generation of AV canal cardiomyocytes (AVCMs) with in vivo-like gene expression and electrophysiological profiles. We created the so-called "assembloids" composed of atrial, AVCM, and ventricular spheroids, which effectively recapitulated unidirectional conduction and the "fast-slow-fast" activation pattern typical for the vertebrate heart. We utilized these systems to reveal intracellular calcium mishandling as the basis of LMNA-associated AV conduction block. In sum, our study introduces novel cell differentiation and tissue construction strategies to facilitate the study of complex disorders affecting heart rhythm.

Design of synthetic microenvironments to promote the maturation of human pluripotent stem cell derived cardiomyocytes

Cardiomyocyte like cells derived from human pluripotent stem cells (hPSC-CMs) have a good application perspective in many fields such as disease modeling, drug screening and clinical treatment. However, these are severely hampered by the fact that hPSC-CMs are immature compared to adult human cardiomyocytes. Therefore, many approaches such as genetic manipulation, biochemical factors supplement, mechanical stress, electrical stimulation and three-dimensional culture have been developed to promote the maturation of hPSC-CMs. Recently, establishing in vitro synthetic artificial microenvironments based on the in vivo development program of cardiomyocytes has achieved much attention due to their inherent properties such as stiffness, plasticity, nanotopography and chemical functionality. In this review, the achievements and deficiency of reported synthetic microenvironments that mainly discussed comprehensive biological, chemical, and physical factors, as well as three-dimensional culture were mainly discussed, which have significance to improve the microenvironment design and accelerate the maturation of hPSC-CMs.

Towards chamber specific heart-on-a-chip for drug testing applications

Modeling of human organs has long been a task for scientists in order to lower the costs of therapeutic development and understand the pathological onset of human disease. For decades, despite marked differences in genetics and etiology, animal models remained the norm for drug discovery and disease modeling. Innovative biofabrication techniques have facilitated the development of organ-on-a-chip technology that has great potential to complement conventional animal models. However, human organ as a whole, more specifically the human heart, is difficult to regenerate in vitro, in terms of its chamber specific orientation and its electrical functional complexity. Recent progress with the development of induced pluripotent stem cell differentiation protocols, made recapitulating the complexity of the human heart possible through the generation of cells representative of atrial & ventricular tissue, the sinoatrial node, atrioventricular node and Purkinje fibers. Current heart-on-a-chip approaches incorporate biological, electrical, mechanical, and topographical cues to facilitate tissue maturation, therefore improving the predictive power for the chamber-specific therapeutic effects targeting adult human. In this review, we will give a summary of current advances in heart-on-a-chip technology and provide a comprehensive outlook on the challenges involved in the development of human physiologically relevant heart-on-a-chip.

Naturally Engineered Maturation of Cardiomyocytes

Ischemic heart disease remains one of the most prominent causes of mortalities worldwide with heart transplantation being the gold-standard treatment option. However, due to the major limitations associated with heart transplants, such as an inadequate supply and heart rejection, there remains a significant clinical need for a viable cardiac regenerative therapy to restore native myocardial function. Over the course of the previous several decades, researchers have made prominent advances in the field of cardiac regeneration with the creation of in vitro human pluripotent stem cell-derived cardiomyocyte tissue engineered constructs. However, these engineered constructs exhibit a functionally immature, disorganized, fetal-like phenotype that is not equivalent physiologically to native adult cardiac tissue. Due to this major limitation, many recent studies have investigated approaches to improve pluripotent stem cell-derived cardiomyocyte maturation to close this large functionality gap between engineered and native cardiac tissue. This review integrates the natural developmental mechanisms of cardiomyocyte structural and functional maturation. The variety of ways researchers have attempted to improve cardiomyocyte maturation in vitro by mimicking natural development, known as natural engineering, is readily discussed. The main focus of this review involves the synergistic role of electrical and mechanical stimulation, extracellular matrix interactions, and non-cardiomyocyte interactions in facilitating cardiomyocyte maturation. Overall, even with these current natural engineering approaches, pluripotent stem cell-derived cardiomyocytes within three-dimensional engineered heart tissue still remain mostly within the early to late fetal stages of cardiomyocyte maturity. Therefore, although the end goal is to achieve adult phenotypic maturity, more emphasis must be placed on elucidating how the in vivo fetal microenvironment drives cardiomyocyte maturation. This information can then be utilized to develop natural engineering approaches that can emulate this fetal microenvironment and thus make prominent progress in pluripotent stem cell-derived maturity toward a more clinically relevant model for cardiac regeneration.

Maturing human pluripotent stem cell-derived cardiomyocytes in human engineered cardiac tissues

Engineering functional human cardiac tissue that mimics the native adult morphological and functional phenotype has been a long held objective. In the last 5 years, the field of cardiac tissue engineering has transitioned from cardiac tissues derived from various animal species to the production of the first generation of human engineered cardiac tissues (hECTs), due to recent advances in human stem cell biology. Despite this progress, the hECTs generated to date remain immature relative to the native adult myocardium. In this review, we focus on the maturation challenge in the context of hECTs, the present state of the art, and future perspectives in terms of regenerative medicine, drug discovery, preclinical safety testing and pathophysiological studies.

Dominant negative consequences of a hERG 1b-specific mutation associated with intrauterine fetal death

The human ether-a-go-go related gene (hERG) encodes two subunits, hERG 1a and hERG 1b, that combine in vivo to conduct the rapid delayed rectifier potassium current (IKr). Reduced IKr slows cardiac action potential (AP) repolarization and is an underlying cause of cardiac arrhythmias associated with long QT syndrome (LQTS). Although the physiological importance of hERG 1b has been elucidated, the effects of hERG 1b disease mutations on cardiac IKr and AP behavior have not been described. To explore the disease mechanism of a 1b-specific mutation associated with a case of intrauterine fetal death, we examined the effects of the 1b-R25W mutation on total protein, trafficking and membrane current levels in HEK293 cells at physiological temperatures. By all measures the 1b-R25W mutation conferred diminished expression, and exerted a temperature-sensitive, dominant-negative effect over the WT hERG 1a protein with which it was co-expressed. Membrane currents were reduced by 60% with no apparent effect on voltage dependence or deactivation kinetics. The dominant-negative effects of R25W were demonstrated in iPSC-CMs, where 1b-R25W transfection diminished native IKr compared to controls. R25W also slowed AP repolarization, and increased AP triangulation and variability in iPSC-CMs, reflecting cellular manifestations of pro-arrhythmia. These data demonstrate that R25W is a dominant-negative mutation with significant pathophysiological consequences, and provide the first direct link between hERG 1b mutation and cardiomyocyte dysfunction.

hERG 1b is critical for human cardiac repolarization

The human ether-à-go-go-related gene (hERG; or KCNH2) encodes the voltage-gated potassium channel underlying IKr, a repolarizing current in the heart. Mutations in KCNH2 or pharmacological agents that reduce IKr slow action potential (AP) repolarization and can trigger cardiac arrhythmias associated with long QT syndrome. Two channel-forming subunits encoded by KCNH2 (hERG 1a and 1b) are expressed in cardiac tissue. In heterologous expression systems, these subunits avidly coassemble and exhibit biophysical and pharmacological properties distinct from those of homomeric hERG 1a channels. Despite these findings, adoption of hERG 1a/1b heteromeric channels as a model for cardiac IKr has been hampered by the lack of evidence for a direct functional role for the 1b subunit in native tissue. In this study, we measured IKr and APs at physiological temperature in cardiomyocytes derived from human induced pluripotent stem cells (iPSC-CMs). We found that specific knockdown of the 1b subunit using shRNA caused reductions in 1b mRNA, 1b protein levels, and IKr magnitude by roughly one-half. AP duration was increased and AP variability was enhanced relative to controls. Early afterdepolarizations, considered cellular substrates for arrhythmia, were also observed in cells with reduced 1b expression. Similar behavior was elicited when channels were effectively converted from heteromers to 1a homomers by expressing a fragment corresponding to the 1a-specific N-terminal Per-Arnt-Sim domain, which is omitted from hERG 1b by alternate transcription. These findings establish that hERG 1b is critical for normal repolarization and that loss of 1b is proarrhythmic in human cardiac cells.

Electrophysiologic properties of the adult human sinus node

Electrophysiologic properties of the sinoatrial (SA) node have been studied extensively in various species. Published studies on human SA nodal properties were carried out on fetal SA node preparations; however, the electrophysiologic properties of adult human cardiac tissues have never been examined. In this study we report, for the first time, recordings of transmembrane potentials in the adult human SA node. Our data demonstrate that the electrical behavior of adult human SA node pacemaker cells resemble those of SA nodal tissue of different animal species. We conclude that the latter ideally mimic the former. Furthermore, human SA node preparations could contribute to a better understanding of some sinus arrhythmias.

Two stable levels of diastolic potential at physiological K+ concentrations in human ventricular myocardial cells

Cells in many specimens of human ventricle can exhibit either of two stable levels of diastolic potential (DP) when exposed to 4 mM K+ in vitro (i.e., -78 +/- 4 mV or -45 +/- 5 mV, mean +/- SEM). In this report we show that the DP of some partially depolarized human ventricular cells developed a sustained 25-35 mV hyperpolarization (n = 28) when bath K+ concentration (K+b) was raised from 4 to 7 mM. On return of K+b to 4 mM, the DP of most, but not all, of these cells returned to the original depolarized levels. In other cells, the transition between the two levels of DP occurred at variable K+b ranging from 1 to 20 mM. We investigated the ionic mechanism(s) underlying the shifts between the two levels of potential by studying the K+ dependence of the DP in partially depolarized cells in 22 specimens of human ventricle. DP hyperpolarized an average of 25.6 mV (from -44.4 +/- 1.3 to -70.0 +/- 1.3 mV; n = 25) when K+b was increased from 4 to 7 mM. Intracellular K+ activity, determined by K+-selective microelectrodes, was within the range of normal reported for other mammalian species (106.7 +/- 4.4 mM in 4 mM K+; n = 22) and was unaffected by increasing K+b to 7 mM (111.7 +/- 6.6 mM; n = 6). Ba2+ (0.05 mM), a blocker of the inward rectifying K+ current, reversibly prevented the hyperpolarization, whereas acetylstrophanthidin (9 microM) failed to inhibit it. These results suggest that the hyperpolarization was due to a K+-dependent increase in K+ permeability and that electrogenic sodium pumping did not contribute significantly to the process. The ionic basis of the depolarization from a hyperpolarized level of DP also was investigated. Decreasing bath Na+ concentration and exposure to 30 microM tetrodotoxin did not prevent the depolarization. However, the depolarization could be inhibited by 2 mM Mn2+. These findings suggest that the depolarization may have been due to a Mn2+-sensitive inward current.

Human atrial conduction with reference to heart rate and refractory periods

Atrial conduction time (ACT) between a stimulating pacemaker electrode and an "impulse-detecting" monophasic action potential (MAP) electrode has been determined in 20 healthy males. A small increase in ACT was found with increasing paced heart rates. Using the extra stimulus technique, ACT was found to increase when the early ectopics approached the atrial effective refractory period (AERP). The degree of ACT prolongation was similar at long and short basic ACTs, indicating that the slowing of impulse propagation probably occurred near the simulating electrode and presumably within 1/2-1 cm from that electrode. The increase in ACT close to the AERP was of importance when determining the atrioventricular refractory period. In three recordings a supernormal phase of conduction was found. The small errors of the MAP recordings in detecting the time of excitation make the method sensitive enough to detect small differences in ACT.

A combined electrophysiological and anatomical study of the human fetal heart

Five human fetal hearts of gestational ages ranging from 12 to 16 weeks were studied with both electrophysiological and anatomical techniques. The electrophysiological behavior of these hearts was comparable with results obtained with other animal species and adult human hearts. This indicates that the fetal heart is functionally mature from an early stage of development; however the tissues are not fully differentiated from an anatomical standpoint. Results of mapping experiments indicated that atrial activation occurred through broad wave fronts, and no electrophysiological evidence was found to support the concept of "specialized internodal conduction." In two hearts, the node and bundle were found to be anatomical contiguity with ventricular myocardium throughout their length, but the conducting tissues were already functionally insulated from the ventricles. The results have significance with regard to concepts of the "sudden death in infancy syndrome" and ventricular pre-excitation.

Membrane response to current pulses in spheroidal aggregates of embryonic heart cells

Hearts from chick embryos aged 4,7, or 14 days were dissociated into their component cells, and the cells allowed to reassociate in the form of smooth-surfaced spheroidal aggregates on a gyratory shaker. Records from intracellular electrodes inserted into two widely spaced cells in a spontaneously beating aggregate indicated that the action potentials occurred virtually simultaneously. In aggregates made quiescent with tetrodotoxin, the voltage response to a current pulse injected in one cell could be noted by recording with a second microelectrode at various distance from the current source. The magnitude of the response was found not to vary with distance. It is concluded that the component cells in an aggregate are normally tightly coupled electrically; the cell boundaries do not constitute an appreciable resistive barrier. Such ag-regates behave as virtually isopential systems, with properties similar to those of single spherical cells, as modeled by Eisenberg and Engel (1970. J. Gen. Physiol. 55:736-757). Passive membrane time constant ranged from 11 to 31 ms, with a mean value of 17 ms; this value did not vary with aggregate size. Input resistance (V/I) varied inversely with aggregate size, as predicted, but with much scatter in the measured values. Specific membrane resistance was calculated as either 13,000 or 800 ohm-cm2 depending on whether input resistance was attributed to the total cell surface membrane area or to the outer surface of the sphere alone. No systematic difference in passive electrical properties of aggregates composed of 4-, 7-, and 14-day cells was seen. It is concluded that these aggregates may be suitable for voltage clamp analysis of their excitable membrane properties.

Electrophysiologic properties of isolated preparations of human atrial myocardium

The electrophysiologic properties of human atrium were studied with intracellular microelectrodes in 22 preparations of human right atrial tissue obtained at the time of corrective open heart surgery. Two types of fibers were identified: the first had electrical characteristics typical of atrial contractile cells and the second those of atrial specialized fibers. Automaticity developed only in the latter type of cell. In 36 impalements of specialized atrial fibers, resting membrane potential was -86 ± 5 mv (mean ±SD), transmembrane action potential amplitude was 102 ± 12 mv, transmembrane action potential overshoot was 16 ± 4 mv. Similar results were obtained in 49 impalements of contractile fibers. Conduction velocity measured 0.28-0.33 m/sec in contractile fibers and 0.41-0.45 m/sec in specialized atrial fibers. Action potential duration of specialized atrial fibers was linearly related to the basic cycle length between 200 and 1000 msec. At cycle lengths greater than 1000 msec the increase in action potential duration was no longer linear. There was a decrease of 58 mv in resting membrane potential of specialized atrial fibers when exposed to a tenfold increase in extracellular [K + ]. This decrease occurred in a linear fashion in [K + ] above 5 mM. These experiments provide reference values for future studies dealing with electrophysiology of human atrial tissue.