Utilizing human induced pluripotent stem cells to study atrial arrhythmias in the short QT syndrome.

BACKGROUND: Among the monogenic inherited causes of atrial fibrillation is the short QT syndrome (SQTS), a rare channelopathy causing atrial and ventricular arrhythmias. One of the limitations in studying the mechanisms and optimizing treatment of SQTS-related atrial arrhythmias has been the lack of relevant human atrial tissues models. OBJECTIVE: To generate a unique model to study SQTS-related atrial arrhythmias by combining the use of patient-specific human induced pluripotent stem cells (hiPSCs), atrial-specific differentiation schemes, two-dimensional tissue modeling, optical mapping, and drug testing. METHODS AND RESULTS: SQTS (N588K KCNH2 mutation), isogenic-control, and healthy-control hiPSCs were coaxed to differentiate into atrial cardiomyocytes using a retinoic-acid based differentiation protocol. The atrial identity of the cells was confirmed by a distinctive pattern of MLC2v downregulation, connexin 40 upregulation, shorter and triangular-shaped action potentials (APs), and expression of the atrial-specific acetylcholine-sensitive potassium current. In comparison to the healthy- and isogenic control cells, the SQTS-hiPSC atrial cardiomyocytes displayed abbreviated APs and refractory periods along with an augmented rapidly activating delayed-rectifier potassium current (IKr). Optical mapping of a hiPSC-based atrial tissue model of the SQTS displayed shortened APD and altered biophysical properties of spiral waves induced in this model, manifested by accelerated spiral-wave frequency and increased rotor curvature. Both AP shortening and arrhythmia irregularities were reversed by quinidine and vernakalant treatment, but not by sotalol. CONCLUSIONS: Patient-specific hiPSC-based atrial cellular and tissue models of the SQTS were established, which provide examples on how this type of modeling can shed light on the pathogenesis and pharmacological treatment of inherited atrial arrhythmias.

shRNAs Targeting a Common KCNQ1 Variant Could Alleviate Long-QT1 Disease Severity by Inhibiting a Mutant Allele.

Long-QT syndrome type 1 (LQT1) is caused by mutations in KCNQ1. Patients heterozygous for such a mutation co-assemble both mutant and wild-type KCNQ1-encoded subunits into tetrameric Kv7.1 potassium channels. Here, we investigated whether allele-specific inhibition of mutant KCNQ1 by targeting a common variant can shift the balance towards increased incorporation of the wild-type allele to alleviate the disease in human-induced pluripotent stem-cell-derived cardiomyocytes (hiPSC-CMs). We identified the single nucleotide polymorphisms (SNP) rs1057128 (G/A) in KCNQ1, with a heterozygosity of 27% in the European population. Next, we determined allele-specificity of short-hairpin RNAs (shRNAs) targeting either allele of this SNP in hiPSC-CMs that carry an LQT1 mutation. Our shRNAs downregulated 60% of the A allele and 40% of the G allele without affecting the non-targeted allele. Suppression of the mutant KCNQ1 allele by 60% decreased the occurrence of arrhythmic events in hiPSC-CMs measured by a voltage-sensitive reporter, while suppression of the wild-type allele increased the occurrence of arrhythmic events. Furthermore, computer simulations based on another LQT1 mutation revealed that 60% suppression of the mutant KCNQ1 allele shortens the prolonged action potential in an adult cardiomyocyte model. We conclude that allele-specific inhibition of a mutant KCNQ1 allele by targeting a common variant may alleviate the disease. This novel approach avoids the need to design shRNAs to target every single mutation and opens up the exciting possibility of treating multiple LQT1-causing mutations with only two shRNAs.

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 I(Kr). 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, K(ATP)-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.

Derivation and cardiomyocyte differentiation of induced pluripotent stem cells from heart failure patients.

AIMS: Myocardial cell replacement therapies are hampered by a paucity of sources for human cardiomyocytes and by the expected immune rejection of allogeneic cell grafts. The ability to derive patient-specific human-induced pluripotent stem cells (hiPSCs) may provide a solution to these challenges. We aimed to derive hiPSCs from heart failure (HF) patients, to induce their cardiomyocyte differentiation, to characterize the generated hiPSC-derived cardiomyocytes (hiPSC-CMs), and to evaluate their ability to integrate with pre-existing cardiac tissue. METHODS AND RESULTS: Dermal fibroblasts from two HF patients were reprogrammed by retroviral delivery of Oct4, Sox2, and Klf4 or by using an excisable polycistronic lentiviral vector. The resulting HF-hiPSCs displayed adequate reprogramming properties and could be induced to differentiate into cardiomyocytes with the same efficiency as control hiPSCs (derived from human foreskin fibroblasts). Gene expression and immunostaining studies confirmed the cardiomyocyte phenotype of the differentiating HF-hiPSC-CMs. Multi-electrode array recordings revealed the development of a functional cardiac syncytium and adequate chronotropic responses to adrenergic and cholinergic stimulation. Next, functional integration and synchronized electrical activities were demonstrated between hiPSC-CMs and neonatal rat cardiomyocytes in co-culture studies. Finally, in vivo transplantation studies in the rat heart revealed the ability of the HF-hiPSC-CMs to engraft, survive, and structurally integrate with host cardiomyocytes. CONCLUSIONS: Human-induced pluripotent stem cells can be established from patients with advanced heart failure and coaxed to differentiate into cardiomyocytes, which can integrate with host cardiac tissue. This novel source for patient-specific heart cells may bring a unique value to the emerging field of cardiac regenerative medicine.

Utilizing Human-Induced Pluripotent Stem Cells to Study Cardiac Electroporation Pulsed-Field Ablation.

BACKGROUND: Electroporation is a promising nonthermal ablation method for cardiac arrhythmia treatment. Although initial clinical studies found electroporation pulsed-field ablation (PFA) both safe and efficacious, there are significant knowledge gaps concerning the mechanistic nature and electrophysiological consequences of cardiomyocyte electroporation, contributed by the paucity of suitable human in vitro models. Here, we aimed to establish and characterize a functional in vitro model based on human-induced pluripotent stem cells (hiPSCs)-derived cardiac tissue, and to study the fundamentals of cardiac PFA. METHODS: hiPSC-derived cardiomyocytes were seeded as circular cell sheets and subjected to different PFA protocols. Detailed optical mapping, cellular, and molecular characterizations were performed to study PFA mechanisms and electrophysiological outcomes. RESULTS: PFA generated electrically silenced lesions within the hiPSC-derived cardiac circular cell sheets, resulting in areas of conduction block. Both reversible and irreversible electroporation components were identified. Significant electroporation reversibility was documented within 5 to 15-minutes post-PFA. Irreversibly electroporated regions persisted at 24-hours post-PFA. Per single pulse, high-frequency PFA was less efficacious than standard (monophasic) PFA, whereas increasing pulse-number augmented lesion size and diminished reversible electroporation. PFA augmentation could also be achieved by increasing extracellular Ca2+ levels. Flow-cytometry experiments revealed that regulated cell death played an important role following PFA. Assessing for PFA antiarrhythmic properties, sustainable lines of conduction block could be generated using PFA, which could either terminate or isolate arrhythmic activity in the hiPSC-derived cardiac circular cell sheets. CONCLUSIONS: Cardiac electroporation may be studied using hiPSC-derived cardiac tissue, providing novel insights into PFA temporal and electrophysiological characteristics, facilitating electroporation protocol optimization, screening for potential PFA-sensitizers, and investigating the mechanistic nature of PFA antiarrhythmic properties.

Optogenetic Control of Human Induced Pluripotent Stem Cell-Derived Cardiac Tissue Models.

Background Optogenetics, using light-sensitive proteins, emerged as a unique experimental paradigm to modulate cardiac excitability. We aimed to develop high-resolution optogenetic approaches to modulate electrical activity in 2- and 3-dimensional cardiac tissue models derived from human induced pluripotent stem cell (hiPSC)-derived cardiomyocytes. Methods and Results To establish light-controllable cardiac tissue models, opsin-carrying HEK293 cells, expressing the light-sensitive cationic-channel CoChR, were mixed with hiPSC-cardiomyocytes to generate 2-dimensional hiPSC-derived cardiac cell-sheets or 3-dimensional engineered heart tissues. Complex illumination patterns were designed with a high-resolution digital micro-mirror device. Optical mapping and force measurements were used to evaluate the tissues' electromechanical properties. The ability to optogenetically pace and shape the tissue's conduction properties was demonstrated by using single or multiple illumination stimulation sites, complex illumination patterns, or diffuse illumination. This allowed to establish in vitro models for optogenetic-based cardiac resynchronization therapy, where the electrical activation could be synchronized (hiPSC-derived cardiac cell-sheets and engineered heart tissue models) and contractile properties improved (engineered heart tissues). Next, reentrant activity (rotors) was induced in the hiPSC-derived cardiac cell-sheets and engineered heart tissue models through optogenetics programmed- or cross-field stimulations. Diffuse illumination protocols were then used to terminate arrhythmias, demonstrating the potential to study optogenetics cardioversion mechanisms and to identify optimal illumination parameters for arrhythmia termination. Conclusions By combining optogenetics and hiPSC technologies, light-controllable human cardiac tissue models could be established, in which tissue excitability can be modulated in a functional, reversible, and localized manner. This approach may bring a unique value for physiological/pathophysiological studies, for disease modeling, and for developing optogenetic-based cardiac pacing, resynchronization, and defibrillation approaches.

Characterization of the mechanism by which a nonsense variant in RYR2 leads to disordered calcium handling.

Heterozygous missense variants of the cardiac ryanodine receptor gene (RYR2) cause catecholaminergic polymorphic ventricular tachycardia (CPVT). These missense variants of RYR2 result in a gain of function of the ryanodine receptors, characterized by increased sensitivity to activation by calcium that results in an increased propensity to develop calcium waves and delayed afterdepolarizations. We have recently detected a nonsense variant in RYR2 in a young patient who suffered an unexplained cardiac arrest. To understand the mechanism by which this variant in RYR2, p.(Arg4790Ter), leads to ventricular arrhythmias, human induced pluripotent stem cells (hiPSCs) harboring the novel nonsense variant in RYR2 were generated and differentiated into cardiomyocytes (RYR2-hiPSC-CMs) and molecular and calcium handling properties were studied. RYR2-hiPSC-CMs displayed significant calcium handling abnormalities at baseline and following treatment with isoproterenol. Treatment with carvedilol and nebivolol resulted in a significant reduction in calcium handling abnormalities in the RYR2-hiPSC-CMs. Expression of the mutant RYR2 allele was confirmed at the mRNA level and partial silencing of the mutant allele resulted in a reduction in calcium handling abnormalities at baseline. The nonsense variant behaves similarly to other gain of function variants in RYR2. Carvedilol and nebivolol may be suitable treatments for patients with gain of function RYR2 variants.

In vivo assessment of the electrophysiological integration and arrhythmogenic risk of myocardial cell transplantation strategies.

Cell replacement strategies are promising interventions aiming to improve myocardial performance. Yet, the electrophysiological impact of these approaches has not been elucidated. We assessed the electrophysiological consequences of grafting of two candidate cell types, that is, skeletal myoblasts and human embryonic stem cell-derived cardiomyocytes (hESC-CMs). The fluorescently labeled (DiO) candidate cells were grafted into the rat's left ventricular myocardium. Two weeks later, optical mapping was performed using the Langendorff-perfused rat heart preparation. Images were obtained with appropriate filters to delineate the heart's anatomy, to identify the DiO-labeled cells, and to associate this information with the voltage-mapping data (using the voltage-sensitive dye PGH-I). Histological examination revealed the lack of gap junctions between grafted skeletal myotubes and host cardiomyocytes. In contrast, positive Cx43 immunostaining was observed between donor and host cardiomyocytes in the hESC-CMs-transplanted hearts. Optical mapping demonstrated either normal conduction (four of six) or minimal conduction slowing (two of six) at the hESC-CMs engraftment sites. In contrast, marked slowing of conduction or conduction block was seen (seven of eight) at the myoblast transplantation sites. Ventricular arrhythmias could not be induced in the hESC-CM hearts following programmed electrical stimulation but were inducible in 50% of the myoblast-engrafted hearts. In summary, a unique method for assessment of the electrophysiological impact of myocardial cell therapy is presented. Our results demonstrate the ability of hESC-CMs to functionally integrate with host tissue. In contrast, transplantation of cells that do not form gap junctions (skeletal myoblats) led to localized conduction disturbances and to the generation of a proarrhythmogenic substrate.

Using Decellularization/Recellularization Processes to Prepare Liver and Cardiac Engineered Tissues.

Tissue engineering provides unique opportunities for disease modeling, drug testing, and regenerative medicine applications. The use of cell-seeded scaffolds to promote tissue development is the hallmark of the tissue engineering. Among the different types of scaffolds (derived from either natural or synthetic polymers) used in the field, the use of decellularized tissues/organs is specifically attractive. The decellularization process involves the removal of native cells from the original tissue, allowing for the preservation of the three-dimensional (3D) macroscopic and microscopic structures of the tissue and extracellular matrix (ECM) composition. Following recellularization, the resulting scaffold provides the seeded cells with the appropriate biological signals and mechanical properties of the original tissue. Here, we describe different methods to create viable scaffolds from decellularized heart and liver as useful tools to study and exploit ECM biological key factors for the generation of engineered tissues with enhanced regenerative properties.

Optogenetic modulation of cardiac action potential properties may prevent arrhythmogenesis in short and long QT syndromes.

Abnormal action potential (AP) properties, as occurs in long or short QT syndromes (LQTS and SQTS, respectively), can cause life-threatening arrhythmias. Optogenetics strategies, utilizing light-sensitive proteins, have emerged as experimental platforms for cardiac pacing, resynchronization, and defibrillation. We tested the hypothesis that similar optogenetic tools can modulate the cardiomyocyte's AP properties, as a potentially novel antiarrhythmic strategy. Healthy control and LQTS/SQTS patient-specific human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) were transduced to express the light-sensitive cationic channel channelrhodopsin-2 (ChR2) or the anionic-selective opsin, ACR2. Detailed patch-clamp, confocal-microscopy, and optical mapping studies evaluated the ability of spatiotemporally defined optogenetic protocols to modulate AP properties and prevent arrhythmogenesis in the hiPSC-CMs cell/tissue models. Depending on illumination timing, light-induced ChR2 activation induced robust prolongation or mild shortening of AP duration (APD), while ACR2 activation allowed effective APD shortening. Fine-tuning these approaches allowed for the normalization of pathological AP properties and suppression of arrhythmogenicity in the LQTS/SQTS hiPSC-CM cellular models. We next established a SQTS-hiPSC-CMs-based tissue model of reentrant-arrhythmias using optogenetic cross-field stimulation. An APD-modulating optogenetic protocol was then designed to dynamically prolong APD of the propagating wavefront, completely preventing arrhythmogenesis in this model. This work highlights the potential of optogenetics in studying repolarization abnormalities and in developing novel antiarrhythmic therapies.

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 alpha-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 (I(Kr) blocker), and chromanol 293B (I(Ks) 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.

Cell therapy for modification of the myocardial electrophysiological substrate.

BACKGROUND: Traditional antiarrhythmic pharmacological therapies are limited by their global cardiac action, low efficacy, and significant proarrhythmic effects. We present a novel approach for the modification of the myocardial electrophysiological substrate using cell grafts genetically engineered to express specific ionic channels. METHODS AND RESULTS: To test the aforementioned concept, we performed ex vivo, in vivo, and computer simulation studies to determine the ability of fibroblasts transfected to express the voltage-sensitive potassium channel Kv1.3 to modify the local myocardial excitable properties. Coculturing of the transfected fibroblasts with neonatal rat ventricular myocyte cultures resulted in a significant reduction (68%) in the spontaneous beating frequency of the cultures compared with baseline values and cocultures seeded with naive fibroblasts. In vivo grafting of the transfected fibroblasts in the rat ventricular myocardium significantly prolonged the local effective refractory period from an initial value of 84+/-8 ms (cycle length, 200 ms) to 154+/-13 ms (P<0.01). Margatoxin partially reversed this effect (effective refractory period, 117+/-8 ms; P<0.01). In contrast, effective refractory period did not change in nontransplanted sites (86+/-7 ms) and was only mildly increased in the animals injected with wild-type fibroblasts (73+/-5 to 88+/-4 ms; P<0.05). Similar effective refractory period prolongation also was found during slower pacing drives (cycle length, 350 to 500 ms) after transplantation of the potassium channels expressing fibroblasts (Kv1.3 and Kir2.1) in pigs. Computer modeling studies confirmed the in vivo results. CONCLUSIONS: Genetically engineered cell grafts, transfected to express potassium channels, can couple with host cardiomyocytes and alter the local myocardial electrophysiological properties by reducing cardiac automaticity and prolonging refractoriness.

Modeling Reentry in the Short QT Syndrome With Human-Induced Pluripotent Stem Cell-Derived Cardiac Cell Sheets.

BACKGROUND: The short QT syndrome (SQTS) is an inherited arrhythmogenic syndrome characterized by abnormal ion channel function, life-threatening arrhythmias, and sudden cardiac death. OBJECTIVES: The purpose of this study was to establish a patient-specific human-induced pluripotent stem cell (hiPSC) model of the SQTS, and to provide mechanistic insights into its pathophysiology and therapy. METHODS: Patient-specific hiPSCs were generated from a symptomatic SQTS patient carrying the N588K mutation in the KCNH2 gene, differentiated into cardiomyocytes, and compared with healthy and isogenic (established by CRISPR/Cas9-based mutation correction) control hiPSC-derived cardiomyocytes (hiPSC-CMs). Patch-clamp was used to evaluate action-potential (AP) and IKr current properties at the cellular level. Conduction and arrhythmogenesis were studied at the tissue level using confluent 2-dimensional hiPSC-derived cardiac cell sheets (hiPSC-CCSs) and optical mapping. RESULTS: Intracellular recordings demonstrated shortened action-potential duration (APD) and abbreviated refractory period in the SQTS-hiPSC-CMs. Similarly, voltage- and AP-clamp recordings revealed increased IKr current density due to attenuated inactivation, primarily in the AP plateau phase. Optical mapping of the SQTS-hiPSC-CCSs revealed shortened APD, impaired APD-rate adaptation, abbreviated wavelength of excitation, and increased inducibility of sustained spiral waves. Phase-mapping analysis revealed accelerated and stabilized rotors manifested by increased rotor rotation frequency, increased rotor curvature, decreased core meandering, and increased rotor complexity. Application of quinidine and disopyramide, but not sotalol, normalized APD and suppressed arrhythmia induction. CONCLUSIONS: A novel hiPSC-based model of the SQTS was established at both the cellular and tissue levels. This model recapitulated the disease phenotype in the culture dish and provided important mechanistic insights into arrhythmia mechanisms in the SQTS and its treatment.

Engineered heart tissue models from hiPSC-derived cardiomyocytes and cardiac ECM for disease modeling and drug testing applications.

Cardiac tissue engineering provides unique opportunities for cardiovascular disease modeling, drug testing, and regenerative medicine applications. To recapitulate human heart tissue, we combined human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) with a chitosan-enhanced extracellular-matrix (ECM) hydrogel, derived from decellularized pig hearts. Ultrastructural characterization of the ECM-derived engineered heart tissues (ECM-EHTs) revealed an anisotropic muscle structure, with embedded cardiomyocytes showing more mature properties than 2D-cultured hiPSC-CMs. Force measurements confirmed typical force-length relationships, sensitivity to extracellular calcium, and adequate ionotropic responses to contractility modulators. By combining genetically-encoded calcium and voltage indicators with laser-confocal microscopy and optical mapping, the electrophysiological and calcium-handling properties of the ECM-EHTs could be studied at the cellular and tissue resolutions. This allowed to detect drug-induced changes in contraction rate (isoproterenol, carbamylcholine), optical signal morphology (E-4031, ATX2, isoproterenol, ouabin and quinidine), cellular arrhythmogenicity (E-4031 and ouabin) and alterations in tissue conduction properties (lidocaine, carbenoxolone and quinidine). Similar assays in ECM-EHTs derived from patient-specific hiPSC-CMs recapitulated the abnormal phenotype of the long QT syndrome and catecholaminergic polymorphic ventricular tachycardia. Finally, programmed electrical stimulation and drug-induced pro-arrhythmia led to the development of reentrant arrhythmias in the ECM-EHTs. In conclusion, a novel ECM-EHT model was established, which can be subjected to high-resolution long-term serial functional phenotyping, with important implications for cardiac disease modeling, drug testing and precision medicine. STATEMENT OF SIGNIFICANCE: One of the main objectives of cardiac tissue engineering is to create an in-vitro muscle tissue surrogate of human heart tissue. To this end, we combined a chitosan-enforced cardiac-specific ECM hydrogel derived from decellularized pig hearts with human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) from healthy-controls and patients with inherited cardiac disorders. We then utilized genetically-encoded calcium and voltage fluorescent indicators coupled with unique optical imaging techniques and force-measurements to study the functional properties of the generated engineered heart tissues (EHTs). These studies demonstrate the unique potential of the new model for physiological and pathophysiological studies (assessing contractility, conduction and reentrant arrhythmias), novel disease modeling strategies ("disease-in-a-dish" approach) for studying inherited arrhythmogenic disorders, and for drug testing applications (safety pharmacology).

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 cardiomyocytes. 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.

Human Induced Pluripotent Stem Cell-Derived Cardiac Cell Sheets Expressing Genetically Encoded Voltage Indicator for Pharmacological and Arrhythmia Studies.

Fulfilling the potential of human induced pluripotent stem cell (hiPSC)-derived cardiomyocytes for studying conduction and arrhythmogenesis requires development of multicellular models and methods for long-term repeated tissue phenotyping. We generated confluent hiPSC-derived cardiac cell sheets (hiPSC-CCSs), expressing the genetically encoded voltage indicator ArcLight. ArcLight-based optical mapping allowed generation of activation and action-potential duration (APD) maps, which were validated by mapping the same hiPSC-CCSs with the voltage-sensitive dye, Di-4-ANBDQBS. ArcLight mapping allowed long-term assessment of electrical remodeling in the hiPSC-CCSs and evaluation of drug-induced conduction slowing (carbenoxolone, lidocaine, and quinidine) and APD prolongation (quinidine and dofetilide). The latter studies also enabled step-by-step depiction of drug-induced arrhythmogenesis ("torsades de pointes in the culture dish") and its prevention by MgSO4 and rapid pacing. Phase-mapping analysis allowed biophysical characterization of spiral waves induced in the hiPSC-CCSs and their termination by electrical cardioversion and overdrive pacing. In conclusion, ArcLight mapping of hiPSC-CCSs provides a powerful tool for drug testing and arrhythmia investigation.

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.

Patient-Specific Drug Screening Using a Human Induced Pluripotent Stem Cell Model of Catecholaminergic Polymorphic Ventricular Tachycardia Type 2.

BACKGROUND: Catecholaminergic polymorphic ventricular tachycardia type 2 (CPVT2) results from autosomal recessive CASQ2 mutations, causing abnormal Ca2+-handling and malignant ventricular arrhythmias. We aimed to establish a patient-specific human induced pluripotent stem cell (hiPSC) model of CPVT2 and to use the generated hiPSC-derived cardiomyocytes to gain insights into patient-specific disease mechanism and pharmacotherapy. METHODS AND RESULTS: hiPSC cardiomyocytes were derived from a CPVT2 patient (D307H-CASQ2 mutation) and from healthy controls. Laser-confocal Ca2+ and voltage imaging showed significant Ca2+-transient irregularities, marked arrhythmogenicity manifested by early afterdepolarizations and triggered arrhythmias, and reduced threshold for store overload-induced Ca2+-release events in the CPVT2-hiPSC cardiomyocytes when compared with healthy control cells. Pharmacological studies revealed the prevention of adrenergic-induced arrhythmias by ß-blockers (propranolol and carvedilol), flecainide, and the neuronal sodium-channel blocker riluzole; a direct antiarrhythmic action of carvedilol (independent of its α/ß-adrenergic blocking activity), flecainide, and riluzole; and suppression of abnormal Ca2+ cycling by the ryanodine stabilizer JTV-519 and carvedilol. Mechanistic insights were gained on the different antiarrhythmic actions of the aforementioned drugs, with carvedilol and JTV-519 (but not flecainide or riluzole) acting primarily through sarcoplasmic reticulum stabilization. Finally, comparable outcomes were found between flecainide and labetalol antiarrhythmic effects in vitro and the clinical results in the same patient. CONCLUSIONS: These results demonstrate the ability of hiPSCs cardiomyocytes to recapitulate CPVT2 disease phenotype and drug response in the culture dish, to provide novel insights into disease and drug therapy mechanisms, and potentially to tailor patient-specific drug therapy.