Gil Arbel

Electromechanical integration of cardiomyocytes derived from human embryonic stem cells

Cell therapy is emerging as a promising strategy for myocardial repair. This approach is hampered, however, by the lack of sources for human cardiac tissue and by the absence of direct evidence for functional integration of donor cells into host tissues. Here we investigate whether cells derived from human embryonic stem (hES) cells can restore myocardial electromechanical properties. Cardiomyocyte cell grafts were generated from hES cells in vitro using the embryoid body differentiating system. This tissue formed structural and electromechanical connections with cultured rat cardiomyocytes. In vivo integration was shown in a large-animal model of slow heart rate. The transplanted hES cell–derived cardiomyocytes paced the hearts of swine with complete atrioventricular block, as assessed by detailed three-dimensional electrophysiological mapping and histopathological examination. These results demonstrate the potential of hES-cell cardiomyocytes to act as a rate-responsive biological pacemaker and for future myocardial regeneration strategies.

Modelling the long QT syndrome with induced pluripotent stem cells

The ability to generate patient-specific human induced pluripotent stem cells (iPSCs) offers a new paradigm for modelling human disease and for individualizing drug testing. Congenital long QT syndrome (LQTS) is a familial arrhythmogenic syndrome characterized by abnormal ion channel function and sudden cardiac death. Here we report the development of a patient/disease-specific human iPSC line from a patient with type-2 LQTS (which is due to the A614V missense mutation in the KCNH2 gene). The generated iPSCs were coaxed to differentiate into the cardiac lineage. Detailed whole-cell patch-clamp and extracellular multielectrode recordings revealed significant prolongation of the action-potential duration in LQTS human iPSC-derived cardiomyocytes (the characteristic LQTS phenotype) when compared to healthy control cells. Voltage-clamp studies confirmed that this action-potential-duration prolongation stems from a significant reduction of the cardiac potassium current IKr. Importantly, LQTS-derived cells also showed marked arrhythmogenicity, characterized by early-after depolarizations and triggered arrhythmias. We then used the LQTS human iPSC-derived cardiac-tissue model to evaluate the potency of existing and novel pharmacological agents that may either aggravate (potassium-channel blockers) or ameliorate (calcium-channel blockers, KATP-channel openers and late sodium-channel blockers) the disease phenotype. Our study illustrates the ability of human iPSC technology to model the abnormal functional phenotype of an inherited cardiac disorder and to identify potential new therapeutic agents. As such, it represents a promising paradigm to study disease mechanisms, optimize patient care (personalized medicine), and aid in the development of new therapies.

Tissue Engineering of Vascularized Cardiac Muscle From Human Embryonic Stem Cells

Transplantation of a tissue-engineered heart muscle represents a novel experimental therapeutic paradigm for myocardial diseases. However, this strategy has been hampered by the lack of sources for human cardiomyocytes and by the scarce vasculature in the ischemic area limiting the engraftment and survival of the transplanted muscle. Beyond the necessity of endothelial capillaries for the delivery of oxygen and nutrients to the grafted muscle tissue, interactions between endothelial and cardiomyocyte cells may also play a key role in promoting cell survival and proliferation. In the present study, we describe the formation of synchronously contracting engineered human cardiac tissue derived from human embryonic stem cells containing endothelial vessel networks. The 3D muscle consisted of cardiomyocytes, endothelial cells (ECs), and embryonic fibroblasts (EmFs). The formed vessels were further stabilized by the presence of mural cells originating from the EmFs. The presence of EmFs decreased EC death and increased EC proliferation. Moreover, the presence of endothelial capillaries augmented cardiomyocyte proliferation and did not hamper cardiomyocyte orientation and alignment. Immunostaining, ultrastructural analysis (using transmission electron microscopy), RT-PCR, pharmacological, and confocal laser calcium imaging studies demonstrated the presence of cardiac-specific molecular, ultrastructural, and functional properties of the generated tissue constructs with synchronous activity mediated by action potential propagation through gap junctions. In summary, this is the first report of the construction of 3D vascularized human cardiac tissue that may have unique applications for studies of cardiac development, function, and tissue replacement therapy.

Cardiomyocyte Differentiation of Human Induced Pluripotent Stem Cells

Background— The ability to derive human induced pluripotent stem (hiPS) cell lines by reprogramming of adult fibroblasts with a set of transcription factors offers unique opportunities for basic and translational cardiovascular research. In the present study, we aimed to characterize the cardiomyocyte differentiation potential of hiPS cells and to study the molecular, structural, and functional properties of the generated hiPS-derived cardiomyocytes. Methods and Results— Cardiomyocyte differentiation of the hiPS cells was induced with the embryoid body differentiation system. Gene expression studies demonstrated that the cardiomyocyte differentiation process of the hiPS cells was characterized by an initial increase in mesoderm and cardiomesoderm markers, followed by expression of cardiac-specific transcription factors and finally by cardiac-specific structural genes. Cells in the contracting embryoid bodies were stained positively for cardiac troponin-I, sarcomeric α-actinin, and connexin-43. Reverse-transcription polymerase chain reaction studies demonstrated the expression of cardiac-specific sarcomeric proteins and ion channels. Multielectrode array recordings established the development of a functional syncytium with stable pacemaker activity and action potential propagation. Positive and negative chronotropic responses were induced by application of isoproterenol and carbamylcholine, respectively. Administration of quinidine, E4031 (IKr blocker), and chromanol 293B (IKs blocker) significantly affected repolarization, as manifested by prolongation of the local field potential duration. Conclusions— hiPS cells can differentiate into myocytes with cardiac-specific molecular, structural, and functional properties. These results, coupled with the potential of this technology to generate patient-specific hiPS lines, hold great promise for the development of in vitro models of cardiac genetic disorders, for drug discovery and testing, and for the emerging field of cardiovascular regenerative medicine.

Mechanism of spontaneous excitability in human embryonic stem cell derived cardiomyocytes

Human embryonic stem cell‐derived cardiomyocytes (hES‐CMs) are thought to recapitulate the embryonic development of heart cells. Given the exciting potential of hES‐CMs as replacement tissue in diseased hearts, we investigated the pharmacological sensitivity and ionic current of mid‐stage hES‐CMs (20–35 days post plating). A high‐resolution microelectrode array was used to assess conduction in multicellular preparations of hES‐CMs in spontaneously contracting embryoid bodies (EBs). TTX (10 μm) dramatically slowed conduction velocity from 5.1 to 3.2 cm s−1 while 100 μm TTX caused complete cessation of spontaneous electrical activity in all EBs studied. In contrast, the Ca2+ channel blockers nifedipine or diltiazem (1 μm) had a negligible effect on conduction. These results suggested a prominent Na+ channel current, and therefore we patch‐clamped isolated cells to record Na+ current and action potentials (APs). We found for isolated hES‐CMs a prominent Na+ current (244 ± 42 pA pF−1 at 0 mV; n= 19), and a hyperpolarization‐activated current (HCN), but no inward rectifier K+ current. In cell clusters, 3 μm TTX induced longer AP interpulse intervals and 10 μm TTX caused cessation of spontaneous APs. In contrast nifedipine (Ca2+ channel block) and 2 mm Cs+ (HCN complete block) induced shorter AP interpulse intervals. In single cells, APs stimulated by current pulses had a maximum upstroke velocity (dV/dtmax) of 118 ± 14 V s−1 in control conditions; in contrast, partial block of Na+ current significantly reduced stimulated dV/dtmax (38 ± 15 V s−1). RT‐PCR revealed NaV1.5, CaV1.2, and HCN‐2 expression but we could not detect Kir2.1. We conclude that hES‐CMs at mid‐range development express prominent Na+ current. The absence of background K+ current creates conditions for spontaneous activity that is sensitive to TTX in the same range of partial block of NaV1.5; thus, the NaV1.5 Na+ channel is important for initiating spontaneous excitability in hES‐derived heart cells.

Identification and selection of cardiomyocytes during human embryonic stem cell differentiation

Human embryonic stem cells (hESC) are pluripotent lines that can differentiate in vitro into cell derivatives of all three germ layers, including cardiomy‐ocytes. Successful application of these unique cells in the areas of cardiovascular research and regenerative medicine has been hampered by difficulties in identifying and selecting specific cardiac progenitor cells from the mixed population of differentiating cells. We report the generation of stable transgenic hESC lines, using lentiviral vectors, and single‐cell clones that express a reporter gene (eGFP) under the transcriptional control of a cardiac‐specific promoter (the human myosin light chain‐2V promoter). Our results demonstrate the appearance of eGFP‐expressing cells during the differentiation of the hESC as embryoid bodies (EBs) that can be identified and sorted using FACS (purity>95%, viability>85%). The eGFP‐expressing cells were stained positively for cardiac‐specific proteins (>93%), expressed cardiac‐specific genes, displayed cardiac‐specific action‐potentials, and could form stable myocardial cell grafts following in vivo cell transplantation. The generation of these transgenic hESC lines may be used to identify and study early cardiac precursors for developmental studies, to robustly quantify the extent of cardiomyocyte differentiation, to label the cells for in vivo grafting, and to allow derivation of purified cell populations of cardiomyocytes for future myocardial cell therapy strategies.—Huber, I., Itzhaki, I., Caspi, O., Arbel, G., Tzukerman, M., Gepstein, A., Habib, M., Yankelson, L., Kehat, I., Gepstein, L. Identification and selection of cardiomy‐ocytes during human embryonic stem cell differentiation. FASEB J. 21, 2551–2563 (2007)

In vitro electrophysiological drug testing using human embryonic stem cell derived cardiomyocytes.

Pro-arrhythmia (development of cardiac arrhythmias as a pharmacological side effect) has become the single most common cause of the withdrawal or restrictions of previously marketed drugs. The development of new medications, free from these side effects, is hampered by the lack of an in vitro assay for human cardiac tissue. We hypothesized that human embryonic stem cell-derived cardiomyocytes (hESC-CMs) assessed with a combination of single cell electrophysiology and microelectrode array (MEA) mapping can serve as a novel model for electrophysiological drug screening. Current-clamp studies revealed that E-4031 and Sotalol (IKr blockers) significantly increased hESC-CMs action potential duration and also induced after-depolarizations (the in vitro correlates of increased arrhythmogenic potential). Multicellular aggregates of hESC-CMs were then analyzed with the MEA technique. Application of class I (Quinidine, Procaineamide) and class III (Sotalol) antiarrhythmic agents, E-4031, and Cisapride (a noncardiogenic agent known to lengthen QT) resulted in dose-dependent prolongation of the corrected field potential duration (cFPD). We next utilized the MEA technique to also assess pharmacological effects on conduction. Activation maps demonstrated significant conduction slowing following administration of Na channel blockers (Quinidine and Propafenone) and of the gap junction blocker (1-heptanol). While most attention has been focused on the prospects of using hESC-derived cardiomyocytes for regenerative medicine, this study highlights the possible utilization of these unique cells also for cardiac electrophysiological studies, drug screening, and target validation.

Calcium Handling in Human Embryonic Stem Cell-Derived Cardiomyocytes

The objective of the current study was to characterize calcium handling in developing human embryonic stem cell‐derived cardiomyocytes (hESC‐CMs). To this end, real‐time polymerase chain reaction (PCR), immunocytochemistry, whole‐cell voltage‐clamp, and simultaneous patch‐clamp/laser scanning confocal calcium imaging and surface membrane labeling with di‐8‐aminonaphthylethenylpridinium were used. Immunostaining studies in the hESC‐CMs demonstrated the presence of the sarcoplasmic reticulum (SR) calcium release channels, ryanodine receptor‐2, and inositol‐1,4,5‐trisphosphate (IP3) receptors. Store calcium function was manifested as action‐potential‐induced calcium transients. Time‐to‐target plots showed that these action‐potential‐initiated calcium transients traverse the width of the cell via a propagated wave of intracellular store calcium release. The hESC‐CMs also exhibited local calcium events (“sparks”) that were localized to the surface membrane. The presence of caffeine‐sensitive intracellular calcium stores was manifested following application of focal, temporally limited puffs of caffeine in three different age groups: early‐stage (with the initiation of beating), intermediate‐stage (10 days post‐beating [dpb]), and late‐stage (30–40 dpb) hESC‐CMs. Calcium store load gradually increased during in vitro maturation. Similarly, ryanodine application decreased the amplitude of the spontaneous calcium transients. Interestingly, the expression and function of an IP3‐releasable calcium pool was also demonstrated in the hESC‐CMs in experiments using caged‐IP3 photolysis and antagonist application (2 μM 2‐Aminoethoxydiphenyl borate). In summary, our study establishes the presence of a functional SR calcium store in early‐stage hESC‐CMs and shows a unique pattern of calcium handling in these cells. This study also stresses the importance of the functional characterization of hESC‐CMs both for developmental studies and for the development of future myocardial cell replacement strategies.

Modeling of catecholaminergic polymorphic ventricular tachycardia with patient-specific human-induced pluripotent stem cells.

OBJECTIVES The goal of this study was to establish a patient-specific human-induced pluripotent stem cells (hiPSCs) model of catecholaminergic polymorphic ventricular tachycardia (CPVT). BACKGROUND CPVT is a familial arrhythmogenic syndrome characterized by abnormal calcium (Ca(2+)) handling, ventricular arrhythmias, and sudden cardiac death. METHODS Dermal fibroblasts were obtained from a CPVT patient due to the M4109R heterozygous point RYR2 mutation and reprogrammed to generate the CPVT-hiPSCs. The patient-specific hiPSCs were coaxed to differentiate into the cardiac lineage and compared with healthy control hiPSCs-derived cardiomyocytes (hiPSCs-CMs). RESULTS Intracellular electrophysiological recordings demonstrated the development of delayed afterdepolarizations in 69% of the CPVT-hiPSCs-CMs compared with 11% in healthy control cardiomyocytes. Adrenergic stimulation by isoproterenol (1 μM) or forskolin (5 μM) increased the frequency and magnitude of afterdepolarizations and also led to development of triggered activity in the CPVT-hiPSCs-CMs. In contrast, flecainide (10 μM) and thapsigargin (10 μM) eliminated all afterdepolarizations in these cells. The latter finding suggests an important role for internal Ca(2+) stores in the pathogenesis of delayed afterdepolarizations. Laser-confocal Ca(2+) imaging revealed significant whole-cell [Ca(2+)] transient irregularities (frequent local and large-storage Ca(2+)-release events, broad and double-humped transients, and triggered activity) in the CPVT cardiomyocytes that worsened with adrenergic stimulation and Ca(2+) overload and improved with beta-blockers. Store-overload-induced Ca(2+) release was also identified in the hiPSCs-CMs and the threshold for such events was significantly reduced in the CPVT cells. CONCLUSIONS This study highlights the potential of hiPSCs for studying inherited arrhythmogenic syndromes, in general, and CPVT specifically. As such, it represents a promising paradigm to study disease mechanisms, optimize patient care, and aid in the development of new therapies.

Calcium handling in human induced pluripotent stem cell derived cardiomyocytes.

Background The ability to establish human induced pluripotent stem cells (hiPSCs) by reprogramming of adult fibroblasts and to coax their differentiation into cardiomyocytes opens unique opportunities for cardiovascular regenerative and personalized medicine. In the current study, we investigated the Ca2+-handling properties of hiPSCs derived-cardiomyocytes (hiPSC-CMs). Methodology/Principal Findings RT-PCR and immunocytochemistry experiments identified the expression of key Ca2+-handling proteins. Detailed laser confocal Ca2+ imaging demonstrated spontaneous whole-cell [Ca2+]i transients. These transients required Ca2+ influx via L-type Ca2+ channels, as demonstrated by their elimination in the absence of extracellular Ca2+ or by administration of the L-type Ca2+ channel blocker nifedipine. The presence of a functional ryanodine receptor (RyR)-mediated sarcoplasmic reticulum (SR) Ca2+ store, contributing to [Ca2+]i transients, was established by application of caffeine (triggering a rapid increase in cytosolic Ca2+) and ryanodine (decreasing [Ca2+]i). Similarly, the importance of Ca2+ reuptake into the SR via the SR Ca2+ ATPase (SERCA) pump was demonstrated by the inhibiting effect of its blocker (thapsigargin), which led to [Ca2+]i transients elimination. Finally, the presence of an IP3-releasable Ca2+ pool in hiPSC-CMs and its contribution to whole-cell [Ca2+]i transients was demonstrated by the inhibitory effects induced by the IP3-receptor blocker 2-Aminoethoxydiphenyl borate (2-APB) and the phosopholipase C inhibitor U73122. Conclusions/Significance Our study establishes the presence of a functional, SERCA-sequestering, RyR-mediated SR Ca2+ store in hiPSC-CMs. Furthermore, it demonstrates the dependency of whole-cell [Ca2+]i transients in hiPSC-CMs on both sarcolemmal Ca2+ entry via L-type Ca2+ channels and intracellular store Ca2+ release.