The ionic mechanism by which catecholamines increase the heart rate is incompletely understood. In this study, we have assessed the roles of sinoatrial node L-type Ca v 1.3 (α 1D ) Ca 2+ channels, phosphorylation of L-type channel regulatory partner protein Rad (Ras-related RGK GTP-binding protein), and cAMP-dependent regulation of hyperpolarization-activated HCN (hyperpolarization-activated cyclic nucleotide-gated) channels. We studied β-adrenergic regulation of heart rate and sinoatrial pacemaker activity in mice lacking Ca v 1.3 channels and in mice expressing dihydropyridine-insensitive L-type Ca v 1.2 channels alone or concomitantly expressing cAMP-insensitive HCN4 subunits in a heart-specific and time-controlled manner. We also studied the chronotropic response to sympathomimetics of sinoatrial pacemaker myocytes under conditions of specific inhibition of cAMP-dependent regulation of HCN4 by the cyclic dinucleotide cyclic di-(3′,5′)-GMP and ablation of PKA (protein kinase A)–dependent phosphorylation of Rad. Mutant mice with knockout of Ca v 1.3 and cAMP-insensitive HCN4 subunits in the heart lacked diurnal variation in heart rate and failed to increase their heart rate after administration of catecholamines or during physical activity. Selective pharmacological inhibition of Ca v 1.3 prevented the enhancement of pacemaker activity by sympathomimetics or by direct activation of adenylate cyclase, as well as by phosphodiesterase inhibitors, when cAMP-dependent regulation of HCN was simultaneously silenced. Upregulation of Ca v 1.3 and HCN-mediated funny current ( I f ) accounted for the total change in diastolic current on activation of β-adrenoceptors, explaining the loss of chronotropic effect of catecholamines. Preventing PKA phosphorylation of Rad abrogated the chronotropic response to sympathomimetics of intact hearts under HCN blockade, or in pacemaker myocytes on preventing cAMP-dependent regulation of HCN4, respectively. PKA phosphorylation of Rad, which disinhibits Ca v 1.3 channels and cAMP-dependent activation of HCN channels, are key effectors in β-adrenergic regulation of cardiac pacemaker activity and can sustain positive chronotropic effects independently. These findings on Rad-mediated regulation of Ca v 1.3 and HCN channels unravel the ionic mechanisms underlying the catecholaminergic acceleration of the heart rate.

Statin therapies, used to lower high blood cholesterol, have been proposed to cause skeletal muscle side effects including myalgia and rhabdomyolysis. The ryanodine receptor 1 (RyR1)/Ca²⁺ release channel in skeletal muscle has recently been identified as an off-target binder of statins. Here, we report the cryo-electron microscopy structure of RyR1 in complex with rosuvastatin. Rosuvastatin occupies a single binding site in the pore region of RyR1 stabilizing the open conformation. In the closed conformation, the rosuvastatin binding site is occupied by the RyR1 auxiliary transmembrane helices (TMx1-2). Structural comparison indicates steric overlap of rosuvastatin with simvastatin (at Sim-1) but not with atorvastatin on RyR1, while all these statins may interfere with the conformational stability of the TMx1-2 helices. In contrast to simvastatin and atorvastatin, rosuvastatin does not exhibit multiple binding sites on RyR1 while similarly promoting RyR1 activation, indicating its potential as a preferable lead structure for developing statins that minimizes adverse side effects on RyR1. Here the authors report the RyR1 calcium channel structure bound to rosuvastatin: rosuvastatin occupies the same binding site as simvastatin to activate the channel. This may provide a structural basis for developing statins with fewer muscle side effects.

INTRODUCTION/AIMS:In dystrophic mice (mdx, a genetic homolog of Duchenne muscular dystrophy: DMD), previous studies showed that mechanical ventilation (MV) induces ventilator-induced diaphragmatic dysfunction (VIDD). However, susceptibility to mechanical stress caused by asynchrony remains unknown. Our aims were to investigate whether MV exacerbates diaphragm vulnerability to eccentric stress and to evaluate the role of RyR1 remodeling and NOX2 activation. METHODS:Male mdx and wild-type (WT) mice (10-12 weeks) were assigned to non-ventilated or 6-h MV groups. Diaphragm strength and susceptibility to eccentric contractions mimicking asynchrony were assessed, along with remodeling of the Ca2+ release channel RyR1. Two preventive strategies were tested: in vivo S107 treatment (RyR1-stabilizing compound) administered before MV; and in vitro incubation of diaphragms previously ventilated with S107 or ebselen (NOX2 inhibitor) just before eccentric stress. RESULTS:MV reduced maximal tetanic force by 20.1% in WT and 27.0% in mdx mice without sarcolemmal injury. In mdx mice MV significantly increased susceptibility to eccentric contraction, causing a 45% greater force deficit and increased sarcolemmal damage. Pretreatment with S107 prevented both MV-induced weakness and eccentric stress susceptibility. After 6 h MV, incubation with S107 or ebselen mitigated susceptibility to eccentric contraction. RyR1 oxidation, phosphorylation, and calstabin1 dissociation induced by MV were exacerbated by eccentric stress; S107 preserved calstabin1 binding, while ebselen reduced RyR1 oxidative modifications. DISCUSSION:MV exacerbates diaphragm vulnerability to mechanical stress in dystrophin-deficient muscle through RyR1 remodeling and NOX2 activation. Preventing asynchrony and targeting RyR1 or NOX2 may represent therapeutic strategies to limit respiratory complications in DMD.

Background Human induced pluripotent stem cell derived cardiomyocytes (hiPSC-CMs) constitute an attractive system for basic research and pharmacologic screening of new molecules of clinical interest. Numerous protocols aiming at differentiating atrial- or ventricular-like cardiomyocytes (hiPSC-CMs) are available. Conversely, only a few are available for obtaining patient-derived sinoatrial node-like pacemaker myocytes (PM-hiPSC-CMs). Here we validate a new protocol to differentiate mature PM-hiPSC-CMs as a model of native sinoatrial node (SAN) myocytes. Methods We generated PM-hiPSC-CMs through a 2D matrix-sandwich method promoting epithelial-to-mesenchymal transition and small molecule-based temporal modulation of Wnt signaling pathway. In addition, we treated our cells with triiodothyronine, dexamethasone and intracellular cyclic AMP (DTA) to enhance expression of proteins involved in intracellular Ca2+ handling. Results Proteomic analyses showed expression of key SAN proteins in DTA-treated PM-hiPSC-CMs. Importantly, expression of proteins related to Ca2+ handling was increased in DTA-treated PM-hiPSC-CMs compared to untreated ones. DTA-treated PM-hiPSC-CMs displayed action potentials, ionic currents and intracellular Ca2+ dynamics typical of native SAN. In addition, pacemaker activity responded to both β-adrenergic and muscarinic stimulation. Conclusions Our data indicate that the differentiation protocol effectively generates PM-hiPSC-CMs with typical native human SAN features. This protocol may serve as a potential approach to generate PM-hiPSC-CMs from patients with history of sinoatrial node disfunction (SND) carrying different mutations in ion channels underlying pacemaking. In addition, these in vitro models of SND could be used for testing long-term vector-based gene therapeutic strategies to handle bradycardia. ### Competing Interest Statement The authors have declared no competing interest.

BACKGROUND:Catecholaminergic polymorphic ventricular tachycardia (CPVT) is a rare inherited arrhythmia, with pathogenic variants in the RYR2 gene responsible for 60% of clinically well-defined CPVT cases. Diagnosis of CPVT often occurs after a major cardiac event, posing a severe threat to the patient's life. A data set of patients with CPVT would improve the diagnosis and treatment of patients with CPVT. METHODS:This review cataloged clinical data on patients with RYR2-related CPVT variants from articles published up to October 2020 from PubMed, Scopus, and Embase. Variants were mapped to the structural domains of RYR2. Differences in the age of onset based on variant location and incidence of CPVT symptoms, and differences in treatment strategies were analyzed. RESULTS:In 221 publications analyzed, 964 patients with CPVT (351 male, 463 female) were identified with 263 RYR2 protein-coding variants and a median age of onset of CPVT of 11 years (interquartile range, 7-14 years). A web app was developed to allow users to query the database and is available at https://markslab-cpvtdb.org. The proportion of patients requiring treatments in addition to β-blockers varied between variants. The age of onset of CPVT differed significantly between RYR2 variants located in different exons, domains, and subdomains. Patients with variants in the core solenoid at the domain level, and the core solenoid (exEF-hand) and channel pore at the subdomain level, tended to have a lower age of onset compared with other regions. CONCLUSIONS:This study compiled a comprehensive data set of CPVT-associated RYR2 variants and their clinical phenotypes. The age of onset in certain domains (core solenoid) and subdomains (core solenoid[exEF-hand], channel pore) tended to be lower compared with other regions. Variability in patient phenotypes, such as age of onset and treatment efficacy, along with structural information on variants, suggests that patients may benefit from personalized interventions based on their variant.

Catecholaminergic polymorphic ventricular tachycardia (CPVT) and autism spectrum disorder (ASD) are increasingly recognized as comorbid conditions, yet their shared molecular mechanisms remain unclear. This study investigates a novel RyR2-R169P mutation identified in a patient diagnosed with both CPVT and ASD, hypothesizing that this mutation drives calcium (Ca2+) dysregulation in cardiac and neuronal cells. Using patient-derived induced pluripotent stem cells, we generated ventricular-like cardiomyocytes and midbrain neurons. In cardiomyocytes, the RyR2-R169P mutation increased diastolic Ca2+ leak, elevated single-channel open probability, and induced arrhythmogenic Ca2+ waves under β-adrenergic stress. Similarly, neurons exhibited abnormal cytosolic Ca2+ levels, enlarged soma size, and a clear trend to disrupted neurotransmitter release, including reduced GABA and elevated L-DOPA and serotonin. RyR2 biochemical analysis showed reduced phosphorylation of RyR2 by CaMKII, increased PKA dependent phosphorylation and dissociation of calstabin2 in neurons. Pharmacological stabilization of RyR2 with S107 normalized Ca2+ handling in both cell types, restored neuronal morphology, and prevented calstabin2 depletion and CaMKII phosphorylation increase. S107 restores normal neurotransmitter release only when treatment starts before neuronal differentiation. Structural modeling revealed that the R169P mutation destabilizes the N-terminal domain of RyR2, priming the channel for pathological Ca2+ leak. These findings establish RyR2-R169P as a dual regulator of Ca2+ homeostasis, directly linking cardiac arrhythmogenesis to neurodevelopmental deficits. Our results highlight RyR2 dysfunction as a shared mechanism in CPVT-ASD comorbidity and propose Rycals as a promising therapeutic candidate for mitigating Ca2+-driven pathologies in both tissues. This work demonstrates the importance of RyR2 functional integrity in neurodevelopmental processes. ### Competing Interest Statement ARM is a board member and owns shares in RyCarma Therapeutics, which is targeting RyR channels for therapeutic purposes. All authors have nothing else to disclose. All authors read and approve the final version of the manuscript, and ensure it is the case. ### Funding Statement This work was supported by grants of INSERM, ANR Musage and Fenice and the Fondation Coeur et Recherche of the French Society of Cardiology. ### Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: This study was conducted in accordance with the Declaration of Helsinki and was approved by the Montpellier Hospital Review Board Committee (approval number 1003-HPS2). I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable. Yes All data produced in the present study are available upon reasonable request to the authors.

RyR1-related myopathies (RyR1-RMs) include a wide range of genetic disorders that result from mutations in the RYR1 gene. Pathogenic variants lead to defective intracellular calcium homeostasis and muscle dysfunction. Fixing intracellular calcium leaks by stabilizing the RyR1 calcium channel has been identified as a promising therapeutic target. Gene therapy via prime editing also holds great promise as it can cure diseases by correcting genetic mutations. However, as more than 700 variants have been identified in the RYR1 gene, a universal treatment would be a more suitable solution for patients. Our investigation into the RyR1-S2843A mutation has yielded promising results. Using a calcium leak assay, we determined that the S2843A mutation was protective when combined with pathogenic mutations and significantly reduced the Ca2+ leak of the RyR1 channel. Our study demonstrated that prime editing can efficiently introduce the protective S2843A mutation. In vitro experiments using the RNA electroporation of the prime editing components in human myoblasts achieved a 31% introduction of this mutation. This article lays the foundation for a new therapeutic approach for RyR1-RM, where a unique once-in-a-lifetime prime editing treatment could potentially be universally applied to all patients with a leaky RyR1 channel.

BACKGROUND:The ionic mechanism by which catecholamines increase the heart rate is incompletely understood. In this study, we have assessed the roles of sinoatrial node L-type Cav1.3 (α1D) Ca2+ channels, phosphorylation of L-type channel regulatory partner protein Rad (Ras-related RGK GTP-binding protein), and cAMP-dependent regulation of hyperpolarization-activated HCN (hyperpolarization-activated cyclic nucleotide-gated) channels. METHODS:We studied β-adrenergic regulation of heart rate and sinoatrial pacemaker activity in mice lacking Cav1.3 channels and in mice expressing dihydropyridine-insensitive L-type Cav1.2 channels alone or concomitantly expressing cAMP-insensitive HCN4 subunits in a heart-specific and time-controlled manner. We also studied the chronotropic response to sympathomimetics of sinoatrial pacemaker myocytes under conditions of specific inhibition of cAMP-dependent regulation of HCN4 by the cyclic dinucleotide cyclic di-(3',5')-GMP and ablation of PKA (protein kinase A)-dependent phosphorylation of Rad. RESULTS:Mutant mice with knockout of Cav1.3 and cAMP-insensitive HCN4 subunits in the heart lacked diurnal variation in heart rate and failed to increase their heart rate after administration of catecholamines or during physical activity. Selective pharmacological inhibition of Cav1.3 prevented the enhancement of pacemaker activity by sympathomimetics or by direct activation of adenylate cyclase, as well as by phosphodiesterase inhibitors, when cAMP-dependent regulation of HCN was simultaneously silenced. Upregulation of Cav1.3 and HCN-mediated funny current (If) accounted for the total change in diastolic current on activation of β-adrenoceptors, explaining the loss of chronotropic effect of catecholamines. Preventing PKA phosphorylation of Rad abrogated the chronotropic response to sympathomimetics of intact hearts under HCN blockade, or in pacemaker myocytes on preventing cAMP-dependent regulation of HCN4, respectively. CONCLUSIONS:PKA phosphorylation of Rad, which disinhibits Cav1.3 channels and cAMP-dependent activation of HCN channels, are key effectors in β-adrenergic regulation of cardiac pacemaker activity and can sustain positive chronotropic effects independently. These findings on Rad-mediated regulation of Cav1.3 and HCN channels unravel the ionic mechanisms underlying the catecholaminergic acceleration of the heart rate.

Despite advances in pharmacologic and procedural therapies, heart failure (HF) and cardiac arrhythmias remain significant global health burdens, highlighting the urgent need for novel therapeutic strategies. Defective Ca2+ handling in cardiac myocytes is recognized as a central pathogenic mechanism underlying both heart failure and atrial and ventricular arrhythmias. In this review, we critically assess the current state of research on Ca2+-handling proteins and their role in causing heart failure and arrhythmias, highlighting therapeutic implications. Recent paradigm-shifting discoveries, clinical trial outcomes, and challenges of targeting Ca2+-handling proteins are examined. As outlined in this review, an improved understanding of the relevant proteins and their differential expression and function in human health and disease is crucial for developing Ca2+ handling-targeted therapeutics that can fundamentally alter the natural history of heart failure and arrhythmias.

Statins lower cholesterol, reducing the risk of heart disease, and are among the most frequently prescribed drugs. Approximately 10% of individuals develop statin-associated muscle symptoms (SAMS; myalgias, rhabdomyolysis, and muscle weakness), often rendering them statin intolerant. The mechanism underlying SAMS remains poorly understood. Patients with mutations in the skeletal muscle ryanodine receptor 1 (RyR1)/calcium release channel can be particularly intolerant of statins. High-resolution structures revealed simvastatin binding sites in the pore region of RyR1. Simvastatin stabilized the open conformation of the pore and activated the RyR1 channel. In a mouse expressing a mutant RyR1-T4709M found in a patient with profound statin intolerance, simvastatin caused muscle weakness associated with leaky RyR1 channels. Cotreatment with a Rycal drug that stabilizes the channel closed state prevented simvastatin-induced muscle weakness. Thus, statin binding to RyR1 can cause SAMS, and patients with RyR1 mutations may represent a high-risk group for statin intolerance.

Ryanodine receptors (RyRs) are intracellular Ca2+ channels essential for muscle contraction. Caffeine, a xanthine derivative, has been known for decades to increase muscle contraction and enhance activation of RyRs by increasing the sensitivity to Ca2+. We previously showed that xanthine, the only physiologically relevant xanthine derivative, also binds to and activates RyR2. Most xanthine derivatives and analogs are safe and widely prescribed, with the most popular being the xanthine oxidoreductase inhibitor allopurinol (~15M yearly prescriptions in USA). We propose that xanthine derivatives and analogs that enhance RyRs activity could be used for lead optimization and eventually for the treatment of the diseases that exhibit decreased muscle contraction and reduced RyRs activity, such as RyR1-related diseases, sarcopenia, and heart failure. Here, we show by cryo-EM that xanthine derivatives, analogs, and other related compounds bind to the xanthine/caffeine binding site and activate RyR1, and identify 4-oxopyrimidine as the minimal motif necessary for such interaction.

S100A1, a small homodimeric EF-hand Ca 2+ -binding protein (~21 kDa), plays an important regulatory role in Ca 2+ signaling pathways involved in various biological functions including Ca 2+ cycling and contractile performance in skeletal and cardiac myocytes. One key target of the S100A1 interactome is the ryanodine receptor (RyR), a huge homotetrameric Ca 2+ release channel (~2.3 MDa) of the sarcoplasmic reticulum. Here, we report cryoelectron microscopy structures of S100A1 bound to RyR1, the skeletal muscle isoform, in absence and presence of Ca 2+ . Ca 2+ -free apo-S100A1 binds beneath the bridging solenoid (BSol) and forms contacts with the junctional solenoid and the shell-core linker of RyR1. Upon Ca 2+ -binding, S100A1 undergoes a conformational change resulting in the exposure of the hydrophobic pocket known to serve as a major interaction site of S100A1. Through interactions of the hydrophobic pocket with RyR1, Ca 2+ -bound S100A1 intrudes deeper into the RyR1 structure beneath BSol than the apo-form and induces sideways motions of the C-terminal BSol region toward the adjacent RyR1 protomer resulting in tighter interprotomer contacts. Interestingly, the second hydrophobic pocket of the S100A1-dimer is largely exposed at the hydrophilic surface making it prone to interactions with the local environment, suggesting that S100A1 could be involved in forming larger heterocomplexes of RyRs with other protein partners. Since S100A1 interactions stabilizing BSol are implicated in the regulation of RyR-mediated Ca 2+ release, the characterization of the S100A1 binding site conserved between RyR isoforms may provide the structural basis for the development of therapeutic strategies regarding treatments of RyR-related disorders.

AbstractBackgroundDuchenne muscular dystrophy (DMD) is an X‐linked disorder characterized by progressive muscle weakness due to the absence of functional dystrophin. DMD patients also develop dilated cardiomyopathy (DCM). We have previously shown that DMD (mdx) mice and a canine DMD model (GRMD) exhibit abnormal intracellular calcium (Ca2+) cycling related to early‐stage pathological remodelling of the ryanodine receptor intracellular calcium release channel (RyR2) on the sarcoplasmic reticulum (SR) contributing to age‐dependent DCM.MethodsHere, we used hiPSC‐CMs from DMD patients selected by Speckle‐tracking echocardiography and canine DMD cardiac biopsies to assess key early‐stage Duchenne DCM features.ResultsDystrophin deficiency was associated with RyR2 remodelling and SR Ca2+ leak (RyR2 Po of 0.03 ± 0.01 for HC vs. 0.16 ± 0.01 for DMD, P < 0.01), which led to early‐stage defects including senescence. We observed higher levels of senescence markers including p15 (2.03 ± 0.75 for HC vs. 13.67 ± 5.49 for DMD, P < 0.05) and p16 (1.86 ± 0.83 for HC vs. 10.71 ± 3.00 for DMD, P < 0.01) in DMD hiPSC‐CMs and in the canine DMD model. The fibrosis was increased in DMD hiPSC‐CMs. We observed cardiac hypocontractility in DMD hiPSC‐CMs. Stabilizing RyR2 pharmacologically by S107 prevented most of these pathological features, including the rescue of the contraction amplitude (1.65 ± 0.06 μm for DMD vs. 2.26 ± 0.08 μm for DMD + S107, P < 0.01). These data were confirmed by proteomic analyses, in particular ECM remodelling and fibrosis.ConclusionsWe identified key cellular damages that are established earlier than cardiac clinical pathology in DMD patients, with major perturbation of the cardiac ECC. Our results demonstrated that cardiac fibrosis and premature senescence are induced by RyR2 mediated SR Ca2+ leak in DMD cardiomyocytes. We revealed that RyR2 is an early biomarker of DMD‐associated cardiac damages in DMD patients. The progressive and later DCM onset could be linked with the RyR2‐mediated increased fibrosis and premature senescence, eventually causing cell death and further cardiac fibrosis in a vicious cycle leading to further hypocontractility as a major feature of DCM. The present study provides a novel understanding of the pathophysiological mechanisms of the DMD‐induced DCM. By targeting RyR2 channels, it provides a potential pharmacological treatment.

Heart failure, the leading cause of mortality and morbidity in the developed world, is characterized by cardiac ryanodine receptor 2 channels that are hyperphosphorylated, oxidized, and depleted of the stabilizing subunit calstabin-2. This results in a diastolic sarcoplasmic reticulum Ca2+ leak that impairs cardiac contractility and triggers arrhythmias. Genetic mutations in ryanodine receptor 2 can also cause Ca2+ leak, leading to arrhythmias and sudden cardiac death. Here, we solved the cryogenic electron microscopy structures of ryanodine receptor 2 variants linked either to heart failure or inherited sudden cardiac death. All are in the primed state, part way between closed and open. Binding of Rycal drugs to ryanodine receptor 2 channels reverts the primed state back towards the closed state, decreasing Ca2+ leak, improving cardiac function, and preventing arrhythmias. We propose a structural-physiological mechanism whereby the ryanodine receptor 2 channel primed state underlies the arrhythmias in heart failure and arrhythmogenic disorders. Ryanodine receptors type 2 (RyR2) are essential for cardiac muscle excitation-contraction coupling. Here, authors show that preferred conformations of RyR2 variants linked either to heart failure or inherited sudden cardiac death are similar, suggesting a common cause and potential treatment.

Malignant hyperthermia (MH) is a life-threatening pharmacogenetic condition triggered by volatile anesthetics, which activate pathogenic RyR1 mutants. The small molecule therapeutic dantrolene has long been used to treat MH. However, the binding site and mechanism of dantrolene remain unclear. Here, we present cryo-EM structures of RyR1 bound to dantrolene and the MH trigger agent 4-chloro-m-cresol (4CmC), revealing the dantrolene and 4CmC binding sites in atomic detail. Dantrolene binds stacked with ATP or ADP in the RY12 domain at the corner of the receptor, inducing a conformational change in this domain which is allosterically coupled to pore closure. Functional analyses revealed that ATP or ADP was required for dantrolene inhibition, and a single point mutation that disrupts the peripheral ATP binding site abolished ATP/ADP-dependent dantrolene inhibition. Strikingly, in the absence of dantrolene, this site selectively binds two ADP molecules, suggesting a possible role in ATP/ADP ratio sensing.

Cardiac complications including acute heart failure, arrhythmias, ischemia, infarction, and myocardial inflammation are well-documented in patients infected with the SARS-CoV-2 virus. With recent studies confirming low viral RNA load in COVID-19 patient heart tissues, it is unclear what are the underlying pathophysiological mechanisms linking viral infection to resulting cardiac complications. Previous clinical studies provide only descriptive accounts with small patient cohorts, and other in vitro laboratory studies use infected human induced pluripotent stem cells (hiPSCs), which are limited in their ability to adequately model adult cardiomyocytes.