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

Novel high-dense microelectrode array based multimodal bioelectronic monitoring system for cardiac arrhythmia re-entry analysis

In recent decades, significant progress has been made in the treatment of heart diseases, particularly in the field of personalized medicine. Despite the development of genetic tests, phenotyping and risk stratification are performed based on clinical findings and invasive in vivo techniques, such as stimulation conduction mapping techniques and programmed ventricular pacing. Consequently, label-free non-invasive in vitro functional analysis systems are urgently needed for more accurate and effective in vitro risk stratification, model-based therapy planning, and clinical safety profile evaluation of drugs. To overcome these limitations, a novel multilayer high-density microelectrode array (HD-MEA), with an optimized configuration of 512 sensing and 4 pacing electrodes on a sensor area of 100 mm2, was developed for the bioelectronic detection of re-entry arrhythmia patterns. Together with a co-developed front-end, we monitored label-free and in parallel cardiac electrophysiology based on field potential monitoring and mechanical contraction using impedance spectroscopy at the same microelectrode. In proof of principle experiments, human induced pluripotent stem cell (hiPS)-derived cardiomyocytes were cultured on HD-MEAs and used to demonstrate the sensitive quantification of contraction strength modulation by cardioactive drugs such as blebbistatin (IC50 = 4.2 μM), omecamtiv and levosimendan. Strikingly, arrhythmia-typical rotor patterns (re-entry) can be induced by optimized electrical stimulation sequences and detected with high spatial resolution. Therefore, we provide a novel cardiac re-entry analysis system as a promising reference point for diagnostic approaches based on in vitro assays using patient-specific hiPS-derived cardiomyocytes.

A red-emitting carborhodamine for monitoring and measuring membrane potential

Biological membrane potentials, or voltages, are a central facet of cellular life. Optical methods to visualize cellular membrane voltages with fluorescent indicators are an attractive complement to traditional electrode-based approaches, since imaging methods can be high throughput, less invasive, and provide more spatial resolution than electrodes. Recently developed fluorescent indicators for voltage largely report changes in membrane voltage by monitoring voltage-dependent fluctuations in fluorescence intensity. However, it would be useful to be able to not only monitor changes but also measure values of membrane potentials. This study discloses a fluorescent indicator which can address both. We describe the synthesis of a sulfonated tetramethyl carborhodamine fluorophore. When this carborhodamine is conjugated with an electron-rich, methoxy (-OMe) containing phenylenevinylene molecular wire, the resulting molecule, CRhOMe, is a voltage-sensitive fluorophore with red/far-red fluorescence. Using CRhOMe, changes in cellular membrane potential can be read out using fluorescence intensity or lifetime. In fluorescence intensity mode, CRhOMe tracks fast-spiking neuronal action potentials (APs) with greater signal-to-noise than state-of-the-art BeRST 1 (another voltage-sensitive fluorophore). CRhOMe can also measure values of membrane potential. The fluorescence lifetime of CRhOMe follows a single exponential decay, substantially improving the quantification of membrane potential values using fluorescence lifetime imaging microscopy (FLIM). The combination of red-shifted excitation and emission, mono-exponential decay, and high voltage sensitivity enable fast FLIM recording of APs in cardiomyocytes. The ability to both monitor and measure membrane potentials with red light using CRhOMe makes it an important approach for studying biological voltages.

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.

Heart-on-a-chip systems: disease modeling and drug screening applications

Cardiovascular disease (CVD) is the leading cause of death worldwide, casting a substantial economic footprint and burdening the global healthcare system. Historically, pre-clinical CVD modeling and therapeutic screening have been performed using animal models. Unfortunately, animal models oftentimes fail to adequately mimic human physiology, leading to a poor translation of therapeutics from pre-clinical trials to consumers. Even those that make it to market can be removed due to unforeseen side effects. As such, there exists a clinical, technological, and economical need for systems that faithfully capture human (patho)physiology for modeling CVD, assessing cardiotoxicity, and evaluating drug efficacy. Heart-on-a-chip (HoC) systems are a part of the broader organ-on-a-chip paradigm that leverages microfluidics, tissue engineering, microfabrication, electronics, and gene editing to create human-relevant models for studying disease, drug-induced side effects, and therapeutic efficacy. These compact systems can be capable of real-time measurements and on-demand characterization of tissue behavior and could revolutionize the drug development process. In this review, we highlight the key components that comprise a HoC system followed by a review of contemporary reports of their use in disease modeling, drug toxicity and efficacy assessment, and as part of multi-organ-on-a-chip platforms. We also discuss future perspectives and challenges facing the field, including a discussion on the role that standardization is expected to play in accelerating the widespread adoption of these platforms.

Optical mapping and optogenetics in cardiac electrophysiology research and therapy: a state-of-the-art review

State-of-the-art innovations in optical cardiac electrophysiology are significantly enhancing cardiac research. A potential leap into patient care is now on the horizon. Optical mapping, using fluorescent probes and high-speed cameras, offers detailed insights into cardiac activity and arrhythmias by analysing electrical signals, calcium dynamics, and metabolism. Optogenetics utilizes light-sensitive ion channels and pumps to realize contactless, cell-selective cardiac actuation for modelling arrhythmia, restoring sinus rhythm, and probing complex cell-cell interactions. The merging of optogenetics and optical mapping techniques for 'all-optical' electrophysiology marks a significant step forward. This combination allows for the contactless actuation and sensing of cardiac electrophysiology, offering unprecedented spatial-temporal resolution and control. Recent studies have performed all-optical imaging ex vivo and achieved reliable optogenetic pacing in vivo, narrowing the gap for clinical use. Progress in optical electrophysiology continues at pace. Advances in motion tracking methods are removing the necessity of motion uncoupling, a key limitation of optical mapping. Innovations in optoelectronics, including miniaturized, biocompatible illumination and circuitry, are enabling the creation of implantable cardiac pacemakers and defibrillators with optoelectrical closed-loop systems. Computational modelling and machine learning are emerging as pivotal tools in enhancing optical techniques, offering new avenues for analysing complex data and optimizing therapeutic strategies. However, key challenges remain including opsin delivery, real-time data processing, longevity, and chronic effects of optoelectronic devices. This review provides a comprehensive overview of recent advances in optical mapping and optogenetics and outlines the promising future of optics in reshaping cardiac electrophysiology and therapeutic strategies.

Human sodium current voltage-dependence at physiological temperature measured by coupling a patch-clamp experiment to a mathematical model

Voltage-gated Na+ channels are crucial to action potential propagation in excitable tissues. Because of the high amplitude and rapid activation of the Na+ current, voltage-clamp measurements are very challenging and are usually performed at room temperature. In this study, we measured Na+ current voltage-dependence in stem cell-derived cardiomyocytes at physiological temperature. While the apparent activation and inactivation curves, measured as the dependence of current amplitude on voltage, fall within the range reported in previous studies, we identified a systematic error in our measurements. This error is caused by the deviation of the membrane potential from the command potential of the amplifier. We demonstrate that it is possible to account for this artifact using computer simulation of the patch-clamp experiment. We obtained surprising results through patch-clamp model optimization: a half-activation of -11.5 mV and a half-inactivation of -87 mV. Although the half-activation deviates from previous research, we demonstrate that this estimate reproduces the conduction velocity dependence on extracellular potassium concentration. KEY POINTS: Voltage-gated Na+ currents play a crucial role in excitable tissues including neurons, cardiac and skeletal muscle. Measurement of Na+ current is challenging because of its high amplitude and rapid kinetics, especially at physiological temperature. We have used the patch-clamp technique to measure human Na+ current voltage-dependence in human induced pluripotent stem cell-derived cardiomyocytes. The patch-clamp data were processed by optimization of the model accounting for voltage-clamp experiment artifacts, revealing a large difference between apparent parameters of Na+ current and the results of the optimization. We conclude that actual Na+ current activation is extremely depolarized in comparison to previous studies. The new Na+ current model provides a better understanding of action potential propagation; we demonstrate that it explains propagation in hyperkalaemic conditions.

Recent advances in pluripotent stem cell-derived cardiac organoids and heart-on-chip applications for studying anti-cancer drug-induced cardiotoxicity

Cardiovascular disease (CVD) caused by anti-cancer drug-induced cardiotoxicity is now the second leading cause of mortality among cancer survivors. It is necessary to establish efficient in vitro models for early predicting the potential cardiotoxicity of anti-cancer drugs, as well as for screening drugs that would alleviate cardiotoxicity during and post treatment. Human induced pluripotent stem cells (hiPSCs) have opened up new avenues in cardio-oncology. With the breakthrough of tissue engineering technology, a variety of hiPSC-derived cardiac microtissues or organoids have been recently reported, which have shown enormous potential in studying cardiotoxicity. Moreover, using hiPSC-derived heart-on-chip for studying cardiotoxicity has provided novel insights into the underlying mechanisms. Herein, we summarize different types of anti-cancer drug-induced cardiotoxicities and present an extensive overview on the applications of hiPSC-derived cardiac microtissues, cardiac organoids, and heart-on-chips in cardiotoxicity. Finally, we highlight clinical and translational challenges around hiPSC-derived cardiac microtissues/organoids/heart-on chips and their applications in anti-cancer drug-induced cardiotoxicity. • Anti-cancer drug-induced cardiotoxicities represent pressing challenges for cancer treatments, and cardiovascular disease is the second leading cause of mortality among cancer survivors. • Newly reported in vitro models such as hiPSC-derived cardiac microtissues/organoids/chips show enormous potential for studying cardio-oncology. • Emerging evidence supports that hiPSC-derived cardiac organoids and heart-on-chip are promising in vitro platforms for predicting and minimizing anti-cancer drug-induced cardiotoxicity.

A red-emitting carborhodamine for monitoring and measuring membrane potential

Biological membrane potentials, or voltages, are a central facet of cellular life. Optical methods to visualize cellular membrane voltages with fluorescent indicators are an attractive complement to traditional electrode-based approaches, since imaging methods can be high throughput, less invasive, and provide more spatial resolution than electrodes. Recently developed fluorescent indicators for voltage largely report changes in membrane voltage by monitoring voltage-dependent fluctuations in fluorescence intensity. However, it would be useful to be able to not only monitor changes, but also measure values of membrane potentials. This study discloses a new fluorescent indicator which can address both. We describe the synthesis of a new sulfonated tetramethyl carborhodamine fluorophore. When this carborhodamine is conjugated with an electron-rich, methoxy (-OMe) containing phenylenevinylene molecular wire, the resulting molecule, CRhOMe, is a voltage-sensitive fluorophore with red/far-red fluorescence. Using CRhOMe, changes in cellular membrane potential can be read out using fluorescence intensity or lifetime. In fluorescence intensity mode, CRhOMe tracks fast-spiking neuronal action potentials with greater signal-to-noise than state-of-the-art BeRST (another voltage-sensitive fluorophore). CRhOMe can also measure values of membrane potential. The fluorescence lifetime of CRhOMe follows a single exponential decay, substantially improving the quantification of membrane potential values using fluorescence lifetime imaging microscopy (FLIM). The combination of red-shifted excitation and emission, mono-exponential decay, and high voltage sensitivity enable fast FLIM recording of action potentials in cardiomyocytes. The ability to both monitor and measure membrane potentials with red light using CRhOMe makes it an important approach for studying biological voltages.

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.

Cardiac involvement in patient-specific induced pluripotent stem cells of myotonic dystrophy type 1: unveiling the impact of voltage-gated sodium channels

Myotonic dystrophy type 1 (DM1) is a genetic disorder that causes muscle weakness and myotonia. In DM1 patients, cardiac electrical manifestations include conduction defects and atrial fibrillation. DM1 results in the expansion of a CTG transcribed into CUG-containing transcripts that accumulate in the nucleus as RNA foci and alter the activity of several splicing regulators. The underlying pathological mechanism involves two key RNA-binding proteins (MBNL and CELF) with expanded CUG repeats that sequester MBNL and alter the activity of CELF resulting in spliceopathy and abnormal electrical activity. In the present study, we identified two DM1 patients with heart conduction abnormalities and characterized their hiPSC lines. Two differentiation protocols were used to investigate both the ventricular and the atrial electrophysiological aspects of DM1 and unveil the impact of the mutation on voltage-gated ion channels, electrical activity, and calcium homeostasis in DM1 cardiomyocytes derived from hiPSCs. Our analysis revealed the presence of molecular hallmarks of DM1, including the accumulation of RNA foci and sequestration of MBNL1 in DM1 hiPSC-CMs. We also observed mis-splicing of SCN5A and haploinsufficiency of DMPK. Furthermore, we conducted separate characterizations of atrial and ventricular electrical activity, conduction properties, and calcium homeostasis. Both DM1 cell lines exhibited reduced density of sodium and calcium currents, prolonged action potential duration, slower conduction velocity, and impaired calcium transient propagation in both ventricular and atrial cardiomyocytes. Notably, arrhythmogenic events were recorded, including both ventricular and atrial arrhythmias were observed in the two DM1 cell lines. These findings enhance our comprehension of the molecular mechanisms underlying DM1 and provide valuable insights into the pathophysiology of ventricular and atrial involvement.

Gene- and variant-specific efficacy of serum/glucocorticoid-regulated kinase 1 inhibition in long QT syndrome types 1 and 2

AIMS

Current long QT syndrome (LQTS) therapy, largely based on beta-blockade, does not prevent arrhythmias in all patients; therefore, novel therapies are warranted. Pharmacological inhibition of the serum/glucocorticoid-regulated kinase 1 (SGK1-Inh) has been shown to shorten action potential duration (APD) in LQTS type 3. We aimed to investigate whether SGK1-Inh could similarly shorten APD in LQTS types 1 and 2.

METHODS AND RESULTS

Human-induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) and hiPSC-cardiac cell sheets (CCS) were obtained from LQT1 and LQT2 patients; CMs were isolated from transgenic LQT1, LQT2, and wild-type (WT) rabbits. Serum/glucocorticoid-regulated kinase 1 inhibition effects (300 nM-10 µM) on field potential durations (FPD) were investigated in hiPSC-CMs with multielectrode arrays; optical mapping was performed in LQT2 CCS. Whole-cell and perforated patch clamp recordings were performed in isolated LQT1, LQT2, and WT rabbit CMs to investigate SGK1-Inh (3 µM) effects on APD. In all LQT2 models across different species (hiPSC-CMs, hiPSC-CCS, and rabbit CMs) and independent of the disease-causing variant (KCNH2-p.A561V/p.A614V/p.G628S/IVS9-28A/G), SGK1-Inh dose-dependently shortened FPD/APD at 0.3-10 µM (by 20-32%/25-30%/44-45%). Importantly, in LQT2 rabbit CMs, 3 µM SGK1-Inh normalized APD to its WT value. A significant FPD shortening was observed in KCNQ1-p.R594Q hiPSC-CMs at 1/3/10 µM (by 19/26/35%) and in KCNQ1-p.A341V hiPSC-CMs at 10 µM (by 29%). No SGK1-Inh-induced FPD/APD shortening effect was observed in LQT1 KCNQ1-p.A341V hiPSC-CMs or KCNQ1-p.Y315S rabbit CMs at 0.3-3 µM.

CONCLUSION

A robust SGK1-Inh-induced APD shortening was observed across different LQT2 models, species, and genetic variants but less consistently in LQT1 models. This suggests a genotype- and variant-specific beneficial effect of this novel therapeutic approach in LQTS.

Applications of synthetic biology in medical and pharmaceutical fields

Synthetic biology aims to design or assemble existing bioparts or bio-components for useful bioproperties. During the past decades, progresses have been made to build delicate biocircuits, standardized biological building blocks and to develop various genomic/metabolic engineering tools and approaches. Medical and pharmaceutical demands have also pushed the development of synthetic biology, including integration of heterologous pathways into designer cells to efficiently produce medical agents, enhanced yields of natural products in cell growth media to equal or higher than that of the extracts from plants or fungi, constructions of novel genetic circuits for tumor targeting, controllable releases of therapeutic agents in response to specific biomarkers to fight diseases such as diabetes and cancers. Besides, new strategies are developed to treat complex immune diseases, infectious diseases and metabolic disorders that are hard to cure via traditional approaches. In general, synthetic biology brings new capabilities to medical and pharmaceutical researches. This review summarizes the timeline of synthetic biology developments, the past and present of synthetic biology for microbial productions of pharmaceutics, engineered cells equipped with synthetic DNA circuits for diagnosis and therapies, live and auto-assemblied biomaterials for medical treatments, cell-free synthetic biology in medical and pharmaceutical fields, and DNA engineering approaches with potentials for biomedical applications.

PITX2 Knockout Induces Key Findings of Electrical Remodeling as Seen in Persistent Atrial Fibrillation

BACKGROUND

Electrical remodeling in human persistent atrial fibrillation is believed to result from rapid electrical activation of the atria, but underlying genetic causes may contribute. Indeed, common gene variants in an enhancer region close to PITX2 (paired-like homeodomain transcription factor 2) are strongly associated with atrial fibrillation, but the mechanism behind this association remains unknown. This study evaluated the consequences of PITX2 deletion (PITX2^-/-) in human induced pluripotent stem cell-derived atrial cardiomyocytes.

METHODS

CRISPR/Cas9 (clustered regularly interspaced short palindromic repeats/clustered regularly interspaced short palindromic repeat-associated 9) was used to delete PITX2 in a healthy human iPSC line that served as isogenic control. Human induced pluripotent stem cell-derived atrial cardiomyocytes were differentiated with unfiltered retinoic acid and cultured in atrial engineered heart tissue. Force and action potential were measured in atrial engineered heart tissues. Single human induced pluripotent stem cell-derived atrial cardiomyocytes were isolated from atrial engineered heart tissue for ion current measurements.

RESULTS

PITX2^-/- atrial engineered heart tissue beats slightly slower than isogenic control without irregularity. Force was lower in PITX2^-/- than in isogenic control (0.053±0.015 versus 0.131±0.017 mN, n=28/3 versus n=28/4, PITX2^-/- versus isogenic control; P<0.0001), accompanied by lower expression of CACNA1C and lower L-type Ca²⁺ current density. Early repolarization was weaker (action potential duration at 20% repolarization; 45.5±13.2 versus 8.6±5.3 ms, n=18/3 versus n=12/4, PITX2^-/- versus isogenic control; P<0.0001), and maximum diastolic potential was more negative (-78.3±3.1 versus -69.7±0.6 mV, n=18/3 versus n=12/4, PITX2^-/- versus isogenic control; P=0.001), despite normal inward rectifier currents (both I_K1 and I_K,ACh) and carbachol-induced shortening of action potential duration.

CONCLUSIONS

Complete PITX2 deficiency in human induced pluripotent stem cell-derived atrial cardiomyocytes recapitulates some findings of electrical remodeling of atrial fibrillation in the absence of fast beating, indicating that these abnormalities could be primary consequences of lower PITX2 levels.

The Advantages, Challenges, and Future of Human-Induced Pluripotent Stem Cell Lines in Type 2 Long QT Syndrome

Type 2 long QT syndrome (LQT2) is the second most common subtype of long QT syndrome and is caused by mutations in KCHN2 encoding the rapidly activating delayed rectifier potassium channel vital for ventricular repolarization. Sudden cardiac death is a sentinel event of LQT2. Preclinical diagnosis by genetic testing is potentially life-saving.Traditional LQT2 models cannot wholly recapitulate genetic and phenotypic features; therefore, there is a demand for a reliable experimental model. Human-induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) meet this challenge. This review introduces the advantages of the hiPSC-CM model over the traditional model and discusses how hiPSC-CM and gene editing are used to decipher mechanisms of LQT2, screen for cardiotoxicity, and identify therapeutic strategies, thus promoting the realization of precision medicine for LQT2 patients.

Portable low-cost macroscopic mapping system for all-optical cardiac electrophysiology

SIGNIFICANCE

All-optical cardiac electrophysiology enables the visualization and control of key parameters relevant to the detection of cardiac arrhythmias. Mapping such responses in human induced pluripotent stem-cell-derived cardiomyocytes (hiPSC-CMs) is of great interest for cardiotoxicity and personalized medicine applications.

AIM

We introduce and validate a very low-cost compact mapping system for macroscopic all-optical electrophysiology in layers of hiPSC-CMs.

APPROACH

The system uses oblique transillumination, low-cost cameras, light-emitting diodes, and off-the-shelf components (total < $ 15 , 000 ) to capture voltage, calcium, and mechanical waves under electrical or optical stimulation.

RESULTS

Our results corroborate the equivalency of electrical and optogenetic stimulation of hiPSC-CMs, and V m - [ Ca 2 + ] i similarity in conduction under pacing. Green-excitable optical sensors are combinable with blue optogenetic actuators (chanelrhodopsin2) only under very low green light ( < 0.05    mW / mm 2 ). Measurements in warmer culture medium yield larger spread of action potential duration and higher conduction velocities compared to Tyrode's solution at room temperature.

CONCLUSIONS

As multiple optical sensors and actuators are combined, our results can help handle the "spectral congestion" and avoid parameter distortion. We illustrate the utility of the system for uncovering the action of cellular uncoupling agents and show extensibility to an epi-illumination mode for future imaging of thicker native or engineered tissues.

Suppression and Replacement Gene Therapy for KCNH2-Mediated Arrhythmias

BACKGROUND

KCNH2-mediated arrhythmia syndromes are caused by loss-of-function (type 2 long QT syndrome [LQT2]) or gain-of-function (type 1 short QT syndrome [SQT1]) pathogenic variants in the KCNH2-encoded K_v11.1 potassium channel, which is essential for the cardiac action potential.

METHODS

A dual-component "suppression-and-replacement" (SupRep) KCNH2 gene therapy was created by cloning into a single construct a custom-designed KCNH2 short hairpin RNA with ~80% knockdown (suppression) and a "short hairpin RNA-immune" KCNH2 cDNA (replacement). Induced pluripotent stem cell-derived cardiomyocytes and their CRISPR-Cas9 variant-corrected isogenic control (IC) induced pluripotent stem cell-derived cardiomyocytes were made for 2 LQT2- (G604S, N633S) and 1 SQT1- (N588K) causative variants. All variant lines were treated with KCNH2-SupRep or non-targeting control short hairpin RNA (shCT). The action potential duration (APD) at 90% repolarization (APD₉₀) was measured using FluoVolt voltage dye.

RESULTS

KCNH2-SupRep achieved variant-independent rescue of both pathologic phenotypes. For LQT2-causative variants, treatment with KCNH2-SupRep resulted in shortening of the pathologically prolonged APD₉₀ to near curative (IC-like) APD₉₀ levels (G604S IC, 471±25 ms; N633S IC, 405±55 ms) compared with treatment with shCT (G604S: SupRep-treated, 452±76 ms versus shCT-treated, 550±41 ms; P<0.0001; N633S: SupRep-treated, 399±105 ms versus shCT-treated, 577±39 ms, P<0.0001). Conversely, for the SQT1-causative variant, N588K, treatment with KCNH2-SupRep resulted in therapeutic prolongation of the pathologically shortened APD₉₀ (IC: 429±16 ms; SupRep-treated: 396±61 ms; shCT-treated: 274±12 ms).

CONCLUSIONS

We provide the first proof-of-principle gene therapy for correction of both LQT2 and SQT1. KCNH2-SupRep gene therapy successfully normalized the pathologic APD₉₀, thereby eliminating the pathognomonic feature of both LQT2 and SQT1.

Modeling the human heart ex vivo-current possibilities and strive for future applications

The high organ specification of the human heart is inversely proportional to its functional recovery after damage. The discovery of induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) has accelerated research in human heart regeneration and physiology. Nevertheless, due to the immaturity of iPSC-CMs, they are far from being an representative model of the adult heart physiology. Therefore, number of laboratories strive to obtain a heart tissues by engineering methods by structuring iPSC-CMs into complex and advanced platforms. By using the iPSC-CMs and arranging them in 3D cultures it is possible to obtain a human heart muscle with physiological capabilities potentially similar to the adult heart, while remaining in vitro. Here, we attempt to describe existing examples of heart muscle either in vitro or ex vivo models and discuss potential options for the further development of such structures. This will be a crucial step for ultimate derivation of complete heart tissue-mimicking organs and their future use in drug development, therapeutic approaches testing, pre-clinical studies, and clinical applications. This review particularly aims to compile available models of advanced human heart tissue for scientists considering which model would best fit their research needs.

Deciphering Common Long QT Syndrome Using CRISPR/Cas9 in Human-Induced Pluripotent Stem Cell-Derived Cardiomyocytes

From carrying potentially pathogenic genes to severe clinical phenotypes, the basic research in the inherited cardiac ion channel disease such as long QT syndrome (LQTS) has been a significant challenge in explaining gene-phenotype heterogeneity. These have opened up new pathways following the parallel development and successful application of stem cell and genome editing technologies. Stem cell-derived cardiomyocytes and subsequent genome editing have allowed researchers to introduce desired genes into cells in a dish to replicate the disease features of LQTS or replace causative genes to normalize the cellular phenotype. Importantly, this has made it possible to elucidate potential genetic modifiers contributing to clinical heterogeneity and hierarchically manage newly identified variants of uncertain significance (VUS) and more therapeutic options to be tested in vitro. In this paper, we focus on and summarize the recent advanced application of human-induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) combined with clustered regularly interspaced short palindromic repeats/CRISPR-associated system 9 (CRISPR/Cas9) in the interpretation for the gene-phenotype relationship of the common LQTS and presence challenges, increasing our understanding of the effects of mutations and the physiopathological mechanisms in the field of cardiac arrhythmias.

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.

Human iPSC-Cardiomyocytes as an Experimental Model to Study Epigenetic Modifiers of Electrophysiology

The epigenetic landscape and the responses to pharmacological epigenetic regulators in each human are unique. Classes of epigenetic writers and erasers, such as histone acetyltransferases, HATs, and histone deacetylases, HDACs, control DNA acetylation/deacetylation and chromatin accessibility, thus exerting transcriptional control in a tissue- and person-specific manner. Rapid development of novel pharmacological agents in clinical testing—HDAC inhibitors (HDACi)—targets these master regulators as common means of therapeutic intervention in cancer and immune diseases. The action of these epigenetic modulators is much less explored for cardiac tissue, yet all new drugs need to be tested for cardiotoxicity. To advance our understanding of chromatin regulation in the heart, and specifically how modulation of DNA acetylation state may affect functional electrophysiological responses, human-induced pluripotent stem-cell-derived cardiomyocyte (hiPSC-CM) technology can be leveraged as a scalable, high-throughput platform with ability to provide patient-specific insights. This review covers relevant background on the known roles of HATs and HDACs in the heart, the current state of HDACi development, applications, and any adverse cardiac events; it also summarizes relevant differential gene expression data for the adult human heart vs. hiPSC-CMs along with initial transcriptional and functional results from using this new experimental platform to yield insights on epigenetic control of the heart. We focus on the multitude of methodologies and workflows needed to quantify responses to HDACis in hiPSC-CMs. This overview can help highlight the power and the limitations of hiPSC-CMs as a scalable experimental model in capturing epigenetic responses relevant to the human heart.

Triiodothyronine and dexamethasone alter potassium channel expression and promote electrophysiological maturation of human-induced pluripotent stem cell-derived cardiomyocytes

BACKGROUND

Human-induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) have emerged as a promising tool for disease modeling and drug development. However, hiPSC-CMs remain functionally immature, which hinders their utility as a model of human cardiomyocytes.

OBJECTIVE

To improve the electrophysiological maturation of hiPSC-CMs.

METHODS AND RESULTS

On day 16 of cardiac differentiation, hiPSC-CMs were treated with 100 nmol/L triiodothyronine (T3) and 1 μmol/L Dexamethasone (Dex) or vehicle for 14 days. On day 30, vehicle- and T3 + Dex-treated hiPSC-CMs were dissociated and replated either as cell sheets or single cells. Optical mapping and patch-clamp technique were used to examine the electrophysiological properties of vehicle- and T3 + Dex-treated hiPSC-CMs. Compared to vehicle, T3 + Dex-treated hiPSC-CMs had a slower spontaneous beating rate, more hyperpolarized resting membrane potential, faster maximal upstroke velocity, and shorter action potential duration. Changes in spontaneous activity and action potential were mediated by decreased hyperpolarization-activated current (I_f) and increased inward rectifier potassium currents (I_K1), sodium currents (I_Na), and the rapidly and slowly activating delayed rectifier potassium currents (I_Kr and I_Ks, respectively). Furthermore, T3 + Dex-treated hiPSC-CM cell sheets (hiPSC-CCSs) exhibited a faster conduction velocity and shorter action potential duration than the vehicle. Inhibition of I_K1 by 100 μM BaCl₂ significantly slowed conduction velocity and prolonged action potential duration in T3 + Dex-treated hiPSC-CCSs but had no effect in the vehicle group, demonstrating the importance of I_K1 for conduction velocity and action potential duration.

CONCLUSION

T3 + Dex treatment is an effective approach to rapidly enhance electrophysiological maturation of hiPSC-CMs.

Genetically Encoded Fluorescent Biosensors for Biomedical Applications

One of the challenges of modern biology and medicine is to visualize biomolecules in their natural environment, in real-time and in a non-invasive fashion, so as to gain insight into their physiological behavior and highlight alterations in pathological settings, which will enable to devise appropriate therapeutic strategies. Genetically encoded fluorescent biosensors constitute a class of imaging agents that enable visualization of biological processes and events directly in situ, preserving the native biological context and providing detailed insight into their localization and dynamics in cells. Real-time monitoring of drug action in a specific cellular compartment, organ, or tissue type; the ability to screen at the single-cell resolution; and the elimination of false-positive results caused by low drug bioavailability that is not detected by in vitro testing methods are a few of the obvious benefits of using genetically encoded fluorescent biosensors in drug screening. This review summarizes results of the studies that have been conducted in the last years toward the fabrication of genetically encoded fluorescent biosensors for biomedical applications with a comprehensive discussion on the challenges, future trends, and potential inputs needed for improving them.

Isogenic Sets of hiPSC-CMs Harboring Distinct KCNH2 Mutations Differ Functionally and in Susceptibility to Drug-Induced Arrhythmias

Mutations in KCNH2 can lead to long QT syndrome type 2. Variable disease manifestation observed with this channelopathy is associated with the location and type of mutation within the protein, complicating efforts to predict patient risk. Here, we demonstrated phenotypic differences in cardiomyocytes derived from isogenic human induced pluripotent stem cells (hiPSC-CMs) genetically edited to harbor mutations either within the pore or tail region of the ion channel. Electrophysiological analysis confirmed that the mutations prolonged repolarization of the hiPSC-CMs, with differences between the mutations evident in monolayer cultures. Blocking the hERG channel revealed that the pore-loop mutation conferred greater susceptibility to arrhythmic events. These findings showed that subtle phenotypic differences related to KCNH2 mutations could be captured by hiPSC-CMs under genetically matched conditions. Moreover, the results support hiPSC-CMs as strong candidates for evaluating the underlying severity of individual KCNH2 mutations in humans, which could facilitate patient risk stratification.

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Mutation-specific differences detected in hiPSC-CMs with same genetic background

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APD and FPD in the hERG pore variant hiPSC-CMs more prolonged than the tail variant

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The pore variant was also more susceptible to drug-induced arrhythmic events

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Potential strategy to determine KCNH2 mutation-specific arrhythmic risk

Scaffold-based and scaffold-free cardiac constructs for drug testing

The safety and therapeutic efficacy of new drugs are tested in experimental animals. However, besides being a laborious, costly process, differences in drug responses between humans and other animals and potential cardiac adverse effects lead to the discontinued development of new drugs. Thus, alternative approaches to animal tests are needed. Cardiotoxicity and responses of human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) to drugs are conventionally evaluated by cell seeding and two-dimensional (2D) culture, which allows measurements of field potential duration and the action potentials of CMs using multielectrode arrays. However, 2D-cultured hiPSC-CMs lack 3D spatial adhesion, and have fewer intercellular and extracellular matrix interactions, as well as different contractile behavior from CMsin vivo. This issue has been addressed using tissue engineering to fabricate three-dimensional (3D) cardiac constructs from hiPSC-CMs culturedin vitro. Tissue engineering can be categorized as scaffold-based and scaffold-free. In scaffold-based tissue engineering, collagen and fibrin gel scaffolds comprise a 3D culture environment in which seeded cells exhibit cardiac-specific functions and drug responses, whereas 3D cardiac constructs fabricated by tissue engineering without a scaffold have high cell density and form intercellular interactions. This review summarizes the characteristics of scaffold-based and scaffold-free cardiac tissue engineering and discusses the applications of fabricated cardiac constructs to drug screening.

The Use of Stem Cell-Derived Organoids in Disease Modeling: An Update

Organoids represent one of the most important advancements in the field of stem cells during the past decade. They are three-dimensional in vitro culturing models that originate from self-organizing stem cells and can mimic the in vivo structural and functional specificities of body organs. Organoids have been established from multiple adult tissues as well as pluripotent stem cells and have recently become a powerful tool for studying development and diseases in vitro, drug screening, and host-microbe interaction. The use of stem cells-that have self-renewal capacity to proliferate and differentiate into specialized cell types-for organoids culturing represents a major advancement in biomedical research. Indeed, this new technology has a great potential to be used in a multitude of fields, including cancer research, hereditary and infectious diseases. Nevertheless, organoid culturing is still rife with many challenges, not limited to being costly and time consuming, having variable rates of efficiency in generation and maintenance, genetic stability, and clinical applications. In this review, we aim to provide a synopsis of pluripotent stem cell-derived organoids and their use for disease modeling and other clinical applications.

A deep learning algorithm to translate and classify cardiac electrophysiology

The development of induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) has been a critical in vitro advance in the study of patient-specific physiology, pathophysiology, and pharmacology. We designed a new deep learning multitask network approach intended to address the low throughput, high variability, and immature phenotype of the iPSC-CM platform. The rationale for combining translation and classification tasks is because the most likely application of the deep learning technology we describe here is to translate iPSC-CMs following application of a perturbation. The deep learning network was trained using simulated action potential (AP) data and applied to classify cells into the drug-free and drugged categories and to predict the impact of electrophysiological perturbation across the continuum of aging from the immature iPSC-CMs to the adult ventricular myocytes. The phase of the AP extremely sensitive to perturbation due to a steep rise of the membrane resistance was found to contain the key information required for successful network multitasking. We also demonstrated successful translation of both experimental and simulated iPSC-CM AP data validating our network by prediction of experimental drug-induced effects on adult cardiomyocyte APs by the latter.

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.

Precision Medicine and cardiac channelopathies: when dreams meet reality

Precision Medicine (PM) is an innovative approach that, by relying on large populations’ datasets, patients’ genetics and characteristics, and advanced technologies, aims at improving risk stratification and at identifying patient-specific management through targeted diagnostic and therapeutic strategies. Cardiac channelopathies are being progressively involved in the evolution brought by PM and some of them are benefiting from these novel approaches, especially the long QT syndrome. Here, we have explored the main layers that should be considered when developing a PM approach for cardiac channelopathies, with a focus on modern in vitro strategies based on patient-specific human-induced pluripotent stem cells and on in silico models. PM is where scientists and clinicians must meet and integrate their expertise to improve medical care in an innovative way but without losing common sense. We have indeed tried to provide the cardiologist’s point of view by comparing state-of-the-art techniques and approaches, including revolutionary discoveries, to current practice. This point matters because the new approaches may, or may not, exceed the efficacy and safety of established therapies. Thus, our own eagerness to implement the most recent translational strategies for cardiac channelopathies must be tempered by an objective assessment to verify whether the PM approaches are indeed making a difference for the patients. We believe that PM may shape the diagnosis and treatment of cardiac channelopathies for years to come. Nonetheless, its potential superiority over standard therapies should be constantly monitored and assessed before translating intellectually rewarding new discoveries into clinical practice.

Bioengineering approaches to mature induced pluripotent stem cell-derived atrial cardiomyocytes to model atrial fibrillation

Induced pluripotent stem cells (iPSCs) serve as a robust platform to model several human arrhythmia syndromes including atrial fibrillation (AF). However, the structural, molecular, functional, and electrophysiological parameters of patient-specific iPSC-derived atrial cardiomyocytes (iPSC-aCMs) do not fully recapitulate the mature phenotype of their human adult counterparts. The use of physiologically inspired microenvironmental cues, such as postnatal factors, metabolic conditioning, extracellular matrix (ECM) modulation, electrical and mechanical stimulation, co-culture with non-parenchymal cells, and 3D culture techniques can help mimic natural atrial development and induce a more mature adult phenotype in iPSC-aCMs. Such advances will not only elucidate the underlying pathophysiological mechanisms of AF, but also identify and assess novel mechanism-based therapies towards supporting a more 'personalized' (i.e. patient-specific) approach to pharmacologic therapy of AF.

Extracellular Vesicles From Epicardial Fat Facilitate Atrial Fibrillation

BACKGROUND

The role of epicardial fat (eFat)-derived extracellular vesicles (EVs) in the pathogenesis of atrial fibrillation (AF) has never been studied. We tested the hypothesis that eFat-EVs transmit proinflammatory, profibrotic, and proarrhythmic molecules that induce atrial myopathy and fibrillation.

METHODS

We collected eFat specimens from patients with (n=32) and without AF (n=30) during elective heart surgery. eFat samples were grown as organ cultures, and the culture medium was collected every 2 days. We then isolated and purified eFat-EVs from the culture medium, and analyzed the EV number, size, morphology, specific markers, encapsulated cytokines, proteome, and microRNAs. Next, we evaluated the biological effects of unpurified and purified EVs on atrial mesenchymal stromal cells and endothelial cells in vitro. To establish a causal association between eFat-EVs and vulnerability to AF, we modeled AF in vitro using induced pluripotent stem cell-derived cardiomyocytes.

RESULTS

Microscopic examination revealed excessive inflammation, fibrosis, and apoptosis in fresh and cultured eFat tissues. Cultured explants from patients with AF secreted more EVs and harbored greater amounts of proinflammatory and profibrotic cytokines, and profibrotic microRNA, as well, than those without AF. The proteomic analysis confirmed the distinctive profile of purified eFat-EVs from patients with AF. In vitro, purified and unpurified eFat-EVs from patients with AF had a greater effect on proliferation and migration of human mesenchymal stromal cells and endothelial cells, compared with eFat-EVs from patients without AF. Last, whereas eFat-EVs from patients with and without AF shortened the action potential duration of induced pluripotent stem cell-derived cardiomyocytes, only eFat-EVs from patients with AF induced sustained reentry (rotor) in induced pluripotent stem cell-derived cardiomyocytes.

CONCLUSIONS

We show, for the first time, a distinctive proinflammatory, profibrotic, and proarrhythmic signature of eFat-EVs from patients with AF. Our findings uncover another pathway by which eFat promotes the development of atrial myopathy and fibrillation.

Considerations for the Bioengineering of Advanced Cardiac In Vitro Models of Myocardial Infarction

Despite the latest advances in cardiovascular biology and medicine, myocardial infarction (MI) remains one of the major causes of deaths worldwide. While reperfusion of the myocardium is critical to limit the ischemic damage typical of a MI event, it causes detrimental morphological and functional changes known as "reperfusion injury." This complex scenario is poorly represented in currently available models of ischemia/reperfusion injury, leading to a poor translation of findings from the bench to the bedside. However, more recent bioengineered in vitro models of the human heart represent more clinically relevant tools to prevent and treat MI in patients. These include 3D cultures of cardiac cells, the use of patient-derived stem cells, and 3D bioprinting technology. This review aims at highlighting the major features typical of a heart attack while comparing current in vitro, ex vivo, and in vivo models. This information has the potential to further guide in developing novel advanced in vitro cardiac models of ischemia/reperfusion injury. It may pave the way for the generation of advanced pathophysiological cardiac models with the potential to develop personalized therapies.

iPSC-Cardiomyocyte Models of Brugada Syndrome-Achievements, Challenges and Future Perspectives

Brugada syndrome (BrS) is an inherited cardiac arrhythmia that predisposes to ventricular fibrillation and sudden cardiac death. It originates from oligogenic alterations that affect cardiac ion channels or their accessory proteins. The main hurdle for the study of the functional effects of those variants is the need for a specific model that mimics the complex environment of human cardiomyocytes. Traditionally, animal models or transient heterologous expression systems are applied for electrophysiological investigations, each of these models having their limitations. The ability to create induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs), providing a source of human patient-specific cells, offers new opportunities in the field of cardiac disease modelling. Contemporary iPSC-CMs constitute the best possible in vitro model to study complex cardiac arrhythmia syndromes such as BrS. To date, thirteen reports on iPSC-CM models for BrS have been published and with this review we provide an overview of the current findings, with a focus on the electrophysiological parameters. We also discuss the methods that are used for cell derivation and data acquisition. In the end, we critically evaluate the knowledge gained by the use of these iPSC-CM models and discuss challenges and future perspectives for iPSC-CMs in the study of BrS and other arrhythmias.

Suppression-Replacement KCNQ1 Gene Therapy for Type 1 Long QT Syndrome

BACKGROUND

Type 1 long QT syndrome (LQT1) is caused by loss-of-function variants in the KCNQ1-encoded K_v7.1 potassium channel α-subunit that is essential for cardiac repolarization, providing the slow delayed rectifier current. No current therapies target the molecular cause of LQT1.

METHODS

A dual-component suppression-and-replacement (SupRep) KCNQ1 gene therapy was created by cloning a KCNQ1 short hairpin RNA and a short hairpin RNA-immune KCNQ1 cDNA modified with synonymous variants in the short hairpin RNA target site, into a single construct. The ability of KCNQ1-SupRep gene therapy to suppress and replace LQT1-causative variants in KCNQ1 was evaluated by means of heterologous expression in TSA201 cells. For a human in vitro cardiac model, induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) were generated from 4 patients with LQT1 (KCNQ1-Y171X, -V254M, -I567S, and -A344A/spl) and an unrelated healthy control. CRISPR-Cas9 corrected isogenic control iPSC-CMs were made for 2 LQT1 lines (correction of KCNQ1-V254M and KCNQ1-A344A/spl). FluoVolt voltage dye was used to measure the cardiac action potential duration (APD) in iPSC-CMs treated with KCNQ1-SupRep.

RESULTS

In TSA201 cells, KCNQ1-SupRep achieved mutation-independent suppression of wild-type KCNQ1 and 3 LQT1-causative variants (KCNQ1-Y171X, -V254M, and -I567S) with simultaneous replacement of short hairpin RNA-immune KCNQ1 as measured by allele-specific quantitative reverse transcription polymerase chain reaction and Western blot. Using FluoVolt voltage dye to measure the cardiac APD in the 4 LQT1 patient-derived iPSC-CMs, treatment with KCNQ1-SupRep resulted in shortening of the pathologically prolonged APD at both 90% and 50% repolarization, resulting in APD values similar to those of the 2 isogenic controls.

CONCLUSIONS

This study provides the first proof-of-principle gene therapy for complete correction of long QT syndrome. As a dual-component gene therapy vector, KCNQ1-SupRep successfully suppressed and replaced KCNQ1 to normal wild-type levels. In TSA201 cells, cotransfection of LQT1-causative variants and KCNQ1-SupRep caused mutation-independent suppression and replacement of KCNQ1. In LQT1 iPSC-CMs, KCNQ1-SupRep gene therapy shortened the APD, thereby eliminating the pathognomonic feature of LQT1.

Recognition of high-specificity hERG K+ channel inhibitor-induced arrhythmia in cardiomyocytes by automated template matching

Cardiovascular disease (CVD) is the number one cause of death in humans. Arrhythmia induced by gene mutations, heart disease, or hERG K⁺ channel inhibitors is a serious CVD that can lead to sudden death or heart failure. Conventional cardiomyocyte-based biosensors can record extracellular potentials and mechanical beating signals. However, parameter extraction and examination by the naked eye are the traditional methods for analyzing arrhythmic beats, and it is difficult to achieve automated and efficient arrhythmic recognition with these methods. In this work, we developed a unique automated template matching (ATM) cardiomyocyte beating model to achieve arrhythmic recognition at the single beat level with an interdigitated electrode impedance detection system. The ATM model was established based on a rhythmic template with a data length that was dynamically adjusted to match the data length of the target beat by spline interpolation. The performance of the ATM model under long-term astemizole, droperidol, and sertindole treatment at different doses was determined. The results indicated that the ATM model based on a random rhythmic template of a signal segment obtained after astemizole treatment presented a higher recognition accuracy (100% for astemizole treatment and 99.14% for droperidol and sertindole treatment) than the ATM model based on arrhythmic multitemplates. We believe this highly specific ATM method based on a cardiomyocyte beating model has the potential to be used for arrhythmia screening in the fields of cardiology and pharmacology.

Drug Screening with Genetically Encoded Fluorescent Sensors: Today and Tomorrow

Genetically-encoded fluorescent sensors have been actively developed over the last few decades and used in live imaging and drug screening. Real-time monitoring of drug action in a specific cellular compartment, organ, or tissue type; the ability to screen at the single-cell resolution; and the elimination of false-positive results caused by low drug bioavailability that is not detected by in vitro testing methods are a few of the obvious benefits of using genetically-encoded fluorescent sensors in drug screening. In combination with high-throughput screening (HTS), some genetically-encoded fluorescent sensors may provide high reproducibility and robustness to assays. We provide a brief overview of successful, perspective, and hopeful attempts at using genetically encoded fluorescent sensors in HTS of modulators of ion channels, Ca2+ homeostasis, GPCR activity, and for screening cytotoxic, anticancer, and anti-parasitic compounds. We discuss the advantages of sensors in whole organism drug screening models and the perspectives of the combination of human disease modeling by CRISPR techniques with genetically encoded fluorescent sensors for drug screening.

Cardiac optogenetics: a decade of enlightenment

The electromechanical function of the heart involves complex, coordinated activity over time and space. Life-threatening cardiac arrhythmias arise from asynchrony in these space-time events; therefore, therapies for prevention and treatment require fundamental understanding and the ability to visualize, perturb and control cardiac activity. Optogenetics combines optical and molecular biology (genetic) approaches for light-enabled sensing and actuation of electrical activity with unprecedented spatiotemporal resolution and parallelism. The year 2020 marks a decade of developments in cardiac optogenetics since this technology was adopted from neuroscience and applied to the heart. In this Review, we appraise a decade of advances that define near-term (immediate) translation based on all-optical electrophysiology, including high-throughput screening, cardiotoxicity testing and personalized medicine assays, and long-term (aspirational) prospects for clinical translation of cardiac optogenetics, including new optical therapies for rhythm control. The main translational opportunities and challenges for optogenetics to be fully embraced in cardiology are also discussed.

Assessment of Cardiotoxicity With Stem Cell-based Strategies

PURPOSE

Adverse cardiovascular drug effects pose a substantial medical risk and represent a common cause of drug withdrawal from the market. Thus, current in vitro assays and in vivo animal models still have shortcomings in assessing cardiotoxicity. A human model for more accurate preclinical cardiotoxicity assessment is highly desirable. Current differentiation protocols allow for the generation of human pluripotent stem cell-derived cardiomyocytes in basically unlimited numbers and offer the opportunity to study drug effects on human cardiomyocytes. The purpose of this review is to provide a brief overview of the current approaches to translate studies with pluripotent stem cell-derived cardiomyocytes from basic science to preclinical risk assessment.

METHODS

A review of the literature was performed to gather data on the pathophysiology of cardiotoxicity, the current cardiotoxicity screening assays, stem cell-derived cardiomyocytes, and their application in cardiotoxicity screening.

FINDINGS

There is increasing evidence that stem cell-derived cardiomyocytes predict arrhythmogenicity with high accuracy. Cardiomyocyte immaturity represents the major limitation so far. However, strategies are being developed to overcome this hurdle, such as tissue engineering. In addition, stem cell-based strategies offer the possibility to assess structural drug toxicity (eg, by anticancer drugs) on complex models that more closely mirror the structure of the heart and contain endothelial cells and fibroblasts.

IMPLICATIONS

Pluripotent stem cell-derived cardiomyocytes have the potential to substantially change how preclinical cardiotoxicity screening is performed. To which extent they will replace or complement current approaches is being evaluated.

Chronic intermittent tachypacing by an optogenetic approach induces arrhythmia vulnerability in human engineered heart tissue

AIMS

Chronic tachypacing is commonly used in animals to induce cardiac dysfunction and to study mechanisms of heart failure and arrhythmogenesis. Human induced pluripotent stem cells (hiPSC) may replace animal models to overcome species differences and ethical problems. Here, 3D engineered heart tissue (EHT) was used to investigate the effect of chronic tachypacing on hiPSC-cardiomyocytes (hiPSC-CMs).

METHODS AND RESULTS

To avoid cell toxicity by electrical pacing, we developed an optogenetic approach. EHTs were transduced with lentivirus expressing channelrhodopsin-2 (H134R) and stimulated by 15 s bursts of blue light pulses (0.3 mW/mm2, 30 ms, 3 Hz) separated by 15 s without pacing for 3 weeks. Chronic optical tachypacing did not affect contractile peak force, but induced faster contraction kinetics, shorter action potentials, and shorter effective refractory periods. This electrical remodelling increased vulnerability to tachycardia episodes upon electrical burst pacing. Lower calsequestrin 2 protein levels, faster diastolic depolarization (DD) and efficacy of JTV-519 (46% at 1 µmol/L) to terminate tachycardia indicate alterations of Ca2+ handling being part of the underlying mechanism. However, other antiarrhythmic compounds like flecainide (69% at 1 µmol/L) and E-4031 (100% at 1 µmol/L) were also effective, but not ivabradine (1 µmol/L) or SEA0400 (10 µmol/L).

CONCLUSION

We demonstrated a high vulnerability to tachycardia of optically tachypaced hiPSC-CMs in EHT and the effective termination by ryanodine receptor stabilization, sodium or hERG potassium channel inhibition. This new model might serve as a preclinical tool to test antiarrhythmic drugs increasing the insight in treating ventricular tachycardia.

Voltage-sensing optical recording: A method of choice for high-throughput assessment of cardiotropic effects

INTRODUCTION

Voltage and calcium-sensing optical recording (VSOR and CSOR, respectively) from human pluripotent stem cell-derived cardiomyocytes (hPSC-CMs) have been validated for in vitro evaluation of cardiotropic effects of drugs. When compared to electrophysiological devices like microelectrode array, multi-well optical recordings present a lower sample rate that may limit their capacity to detect fast depolarization or propagation velocity alterations. Additionally, the respective sensitivities of VSOR and CSOR to different cardiac electrophysiological effects have not been compared in the same conditions.

METHODS

FluoVolt and Cal520 dyes were used in 96 well format on hPSC-CMs to report sodium channel block by lidocaine and propagation slowing by the junctional uncoupler carbenoxolone at three recording frequencies (60, 120 and 200 Hz) as well as their sensitivity to early and late repolarization delay.

RESULTS

Sodium channel block led to a dose-dependent decrease of the VSOR signal rising slope that was improved by an increased sampling frequency. In contrast, the CSOR signal rising slope was only decreased at the highest concentration with no influence from the sampling rate. A similar result was obtained with carbenoxolone. Early repolarization delay by Bay K8644 showed the same effects on VSOR and CSOR signal durations while repolarization slowing by dofetilide had a significantly stronger prolongating effect on the VSOR signal at the lowest concentration.

DISCUSSION

VSOR showed a higher capacity to detect sodium channel block, propagation slowing and modest late repolarization delay than CSOR. Increasing the sampling rate improved the detection threshold of VSOR for excitability and conduction velocity alterations.

Advances in Stem Cell Modeling of Dystrophin-Associated Disease: Implications for the Wider World of Dilated Cardiomyopathy

Familial dilated cardiomyopathy (DCM) is mostly caused by mutations in genes encoding cytoskeletal and sarcomeric proteins. In the pediatric population, DCM is the predominant type of primitive myocardial disease. A severe form of DCM is associated with mutations in the DMD gene encoding dystrophin, which are the cause of Duchenne Muscular Dystrophy (DMD). DMD-associated cardiomyopathy is still poorly understood and orphan of a specific therapy. In the last 5 years, a rise of interest in disease models using human induced pluripotent stem cells (hiPSCs) has led to more than 50 original studies on DCM models. In this review paper, we provide a comprehensive overview on the advances in DMD cardiomyopathy disease modeling and highlight the most remarkable findings obtained from cardiomyocytes differentiated from hiPSCs of DMD patients. We will also describe how hiPSCs based studies have contributed to the identification of specific myocardial disease mechanisms that may be relevant in the pathogenesis of DCM, representing novel potential therapeutic targets.

Efficient differentiation of human pluripotent stem cells into cardiomyocytes on cell sorting thermoresponsive surface

The current differentiation process of human pluripotent stem cells (hPSCs) into cardiomyocytes to enhance the purity of hPSC-derived cardiomyocytes requires some purification processes, which are laborious processes. We developed cell sorting plates, which are prepared from coating thermoresponsive poly(N-isopropylacrylamide) and extracellular matrix proteins. After hPSCs were induced into cardiomyocytes on the thermoresponsive surface coated with laminin-521 for 15 days, the temperature of the cell culture plates was decreased to 8-9 °C to detach the cells partially from the thermoresponsive surface. The detached cells exhibited a higher cardiomyocyte marker of cTnT than the remaining cells on the thermoresponsive surface as well as the cardiomyocytes after purification using conventional cell selection. The detached cells expressed several cardiomyocyte markers, such as α-actinin, MLC2a and NKX2.5. This study suggested that the purification of hPSC-derived cardiomyocytes using cell sorting plates with the thermoresponsive surface is a promising method for the purification of hPSC-derived cardiomyocytes without conventional laborious processes.

Multicellular In vitro Models of Cardiac Arrhythmias: Focus on Atrial Fibrillation

Atrial fibrillation (AF) is the most common cardiac arrhythmia in clinical practice with a large socioeconomic impact due to its associated morbidity, mortality, reduction in quality of life and health care costs. Currently, antiarrhythmic drug therapy is the first line of treatment for most symptomatic AF patients, despite its limited efficacy, the risk of inducing potentially life-threating ventricular tachyarrhythmias as well as other side effects. Alternative, in-hospital treatment modalities consisting of electrical cardioversion and invasive catheter ablation improve patients' symptoms, but often have to be repeated and are still associated with serious complications and only suitable for specific subgroups of AF patients. The development and progression of AF generally results from the interplay of multiple disease pathways and is accompanied by structural and functional (e.g., electrical) tissue remodeling. Rational development of novel treatment modalities for AF, with its many different etiologies, requires a comprehensive insight into the complex pathophysiological mechanisms. Monolayers of atrial cells represent a simplified surrogate of atrial tissue well-suited to investigate atrial arrhythmia mechanisms, since they can easily be used in a standardized, systematic and controllable manner to study the role of specific pathways and processes in the genesis, perpetuation and termination of atrial arrhythmias. In this review, we provide an overview of the currently available two- and three-dimensional multicellular in vitro systems for investigating the initiation, maintenance and termination of atrial arrhythmias and AF. This encompasses cultures of primary (animal-derived) atrial cardiomyocytes (CMs), pluripotent stem cell-derived atrial-like CMs and (conditionally) immortalized atrial CMs. The strengths and weaknesses of each of these model systems for studying atrial arrhythmias will be discussed as well as their implications for future studies.

Circulating re-entrant waves promote maturation of hiPSC-derived cardiomyocytes in self-organized tissue ring

Directed differentiation methods allow acquisition of high-purity cardiomyocytes differentiated from human induced pluripotent stem cells (hiPSCs); however, their immaturity characteristic limits their application for drug screening and regenerative therapy. The rapid electrical pacing of cardiomyocytes has been used for efficiently promoting the maturation of cardiomyocytes, here we describe a simple device in modified culture plate on which hiPSC-derived cardiomyocytes can form three-dimensional self-organized tissue rings (SOTRs). Using calcium imaging, we show that within the ring, reentrant waves (ReWs) of action potential spontaneously originated and ran robustly at a frequency up to 4 Hz. After 2 weeks, SOTRs with ReWs show higher maturation including structural organization, increased cardiac-specific gene expression, enhanced Ca2+-handling properties, an increased oxygen-consumption rate, and enhanced contractile force. We subsequently use a mathematical model to interpret the origination, propagation, and long-term behavior of the ReWs within the SOTRs.

Voltage Imaging of Cardiac Cells and Tissue Using the Genetically Encoded Voltage Sensor Archon1

Precise measurement of action potentials (APs) is needed to observe electrical activity and cellular communication within cardiac tissue. Voltage-sensitive dyes (VSDs) are traditionally used to measure cardiac APs; however, they require acute chemical addition that prevents chronic imaging. Genetically encoded voltage indicators (GEVIs) enable long-term studies of APs without the need of chemical additions, but current GEVIs used in cardiac tissue exhibit poor kinetics and/or low signal to noise (SNR). Here, we demonstrate the use of Archon1, a recently developed GEVI, in hiPSC-derived cardiomyocytes (CMs). When expressed in CMs, Archon1 demonstrated fast kinetics comparable with patch-clamp electrophysiology and high SNR significantly greater than the VSD Di-8-ANEPPS. Additionally, Archon1 enabled monitoring of APs across multiple cells simultaneously in 3D cardiac tissues. These results highlight Archon1's capability to investigate the electrical activity of CMs in a variety of applications and its potential to probe functionally complex in vitro models, as well as in vivo systems.

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Genetic sensor Archon1 reports membrane voltage in hiPSC-derived cardiomyocytes

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Archon1 monitors action potentials in 2D and 3D cardiac tissue with high sensitivity

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Archon1 repeatedly monitored voltage in the same cells and over extended time periods

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Voltage dynamics of multiple cells were recorded simultaneously with Archon1

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 I_Kr 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 I_Kr 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.