Rationale Human induced pluripotent stem cell derived cardiomyocytes (hiPSC-CM) are increasingly being used for modeling heart disease and are under development for regeneration of the injured heart. However, incomplete structural and functional maturation of hiPSC-CM including lack of t-tubules, immature excitation-contraction (EC) coupling, and inefficient Ca-induced Ca release (CICR) remain major limitations. Objective Thyroid and glucocorticoid hormones are critical for heart maturation. We hypothesized that their addition to standard protocols would promote t-tubule development and mature EC coupling of hiPSC-CM when cultured on extracellular matrix with physiological stiffness (Matrigel mattress). Methods and Results HiPSC-CM were generated using a standard chemical differentiation method supplemented with triiodo-L-thyronine (T3) and/or dexamethasone (Dex) during days 16–30 followed by single-cell culture for 5 days on Matrigel mattress. HiPSC-CM treated with T3+Dex, but not with either T3 or Dex alone, developed an extensive t-tubule network. Notably, Matrigel mattress was necessary for t-tubule formation. Compared to adult human ventricular CM, t-tubules in T3+Dex-treated hiPSC-CM were less organized and had more longitudinal elements. Confocal line scans demonstrated spatially and temporally uniform Ca release that is characteristic of EC coupling in the heart ventricle. T3+Dex enhanced elementary Ca release measured by Ca sparks as well as promoted ryanodine receptor (RyR2) structural organization. Simultaneous measurements of L-type Ca current and intracellular Ca release confirmed enhanced functional coupling between L-type Ca channels and RyR2 in T3+Dex cells. Conclusions Our results suggest a permissive role of combined thyroid and glucocorticoid hormones during the cardiac differentiation process which, when coupled with further maturation on Matrigel mattress, is sufficient for t-tubule development, enhanced CICR, and more ventricular-like EC coupling. This new hormone maturation method could advance the utility of hiPSC-CM for disease modeling and cell-based therapy.

Background Calmodulin (CaM) is encoded by three genes, CALM1, CALM2, and CALM3, all of which harbor pathogenic variants linked to long QT syndrome (LQTS) with early and severe expressivity. These LQTS-causative variants reduce CaM affinity to Ca2+ and alter the properties of the cardiac L-type calcium channel (CaV1.2). CaM also modulates NaV1.5 and the ryanodine receptor, RyR2. All of these interactions may play a role in disease pathogenesis. Here, we determine the spectrum and prevalence of pathogenic CaM variants in a cohort of genetically elusive LQTS, and functionally characterize the novel variants. Methods and Results Thirty-nine genetically elusive LQTS cases underwent whole exome sequencing to identify CaM variants. Non-synonymous CaM variants were overrepresented significantly in this heretofore LQTS cohort (15.4%) compared to exome aggregation consortium (0.04%; p<0.0001). When the clinical sequelae of these 6 CaM-positive cases was compared to the 33 CaM-negative cases, CaM-positive cases had a more severe phenotype with an average age of onset of 8 months, an average QTc of 679 ms, and a high prevalence of cardiac arrest. Functional characterization of one novel variant, E141G-CaM, revealed an 11-fold reduction in Ca2+ binding affinity and a functionally-dominant loss of inactivation in CaV1.2, mild accentuation in NaV1.5 late current, but no effect on intracellular RyR2-mediated calcium release. Conclusions Overall, 15% of our genetically elusive LQTS cohort harbored non-synonymous variants in CaM. Genetic testing of CALM1-3 should be pursued for individuals with LQTS, especially those with early childhood cardiac arrest, extreme QT prolongation, and a negative family history.

Background Calmodulin (CaM) mutations are associated with severe forms of long QT syndrome (LQTS) and catecholaminergic polymorphic ventricular tachycardia (CPVT). We recently reported that CaM mutations were found in 13% of genotype-negative LQTS patients, but the prevalence of CaM mutations in genotype-negative CPVT patients is unknown. Here, we identify and characterize CaM mutations in 12 patients with genotype-negative but clinically-diagnosed CPVT. Methods and Results Mutational analysis of CALM1, CALM2 and CALM3 coding regions, in vitro measurement of CaM-Ca2+ (Ca) binding affinity, RyR2-CaM binding, Ca handling, L-type Ca current (LTCC) and action potential duration (APD). We identified a novel CaM mutation – A103V – in CALM3 in 1 of 12 patients (8%), a female who experienced episodes of exertion-induced syncope since age 10, had normal QT interval, and displayed ventricular ectopy during stress testing consistent with CPVT. A103V modestly lowered CaM Ca-binding affinity (3-fold reduction vs WT-CaM), but did not alter CaM binding to RyR2. In permeabilized cardiomyocytes, A103V-CaM (100 nM) promoted spontaneous Ca wave and spark activity, a cellular phenotype of RyR2 activation. Even a 1:3 mixture of A103V-CaM:WT-CaM activated Ca waves, demonstrating functional dominance. Compared to LQTS D96V-CaM, A103V-CaM had significantly less effects on LTCC inactivation and APD, and caused delayed after depolarizations (DADs) and triggered beats in intact cardiomyocytes. Conclusions We discovered a novel CPVT mutation in the CALM3 gene that shares functional characteristics with established CPVT-associated mutations in CALM1. A small proportion of A103V-CaM is sufficient to evoke arrhythmogenic Ca disturbances via RyR2 dysregulation, which explains the autosomal dominant inheritance.

Dantrolene is the first line therapy of malignant hyperthermia. Animal studies suggest that dantrolene also protects against heart failure and arrhythmias caused by spontaneous Ca 21 release. Although dantrolene inhibits Ca 21 release from the sarcoplasmic reticulum of skeletal and cardiac muscle preparations, its mechanism of action has remained controversial, because dantrolene does not inhibit single ryanodine receptor (RyR) Ca 21 release channels in lipid bilayers. Here we test the hypothesis that calmodulin (CaM), a physiologic RyR binding partner that is lost during incorporation into lipid bilayers, is required for dantrolene inhibition of RyR channels. In single channel recordings (100 nM cytoplasmic [Ca 21 ] 1 2 mM ATP), dantrolene caused inhibition of RyR1 (rabbit skeletal muscle) and RyR2 (sheep) with a maximal inhibition of P o (E max ) to 52 6 4% of control only after adding physiologic [CaM] 5 100 nM. Dantrolene inhibited RyR2 with an IC 50 of 0.16 6 0.03 mM. Mutant N98S-CaM facilitated dantrolene inhibition with an IC 50 5 5.9 6 0.3 nM. In mouse cardiomyocytes, dantrolene had no effect on cardiac Ca 21 release in the absence of CaM, but reduced Ca 21 wave frequency (IC 50 5 0.42 6 0.18 mM, E max 5 47 6 4%) and amplitude (IC 50 5 0.19 6 0.04 mM, E max 5 66 6 4%) in the presence of 100 nM CaM. We conclude that CaM is essential for dantrolene inhibition of RyR1 and RyR2. Its absence explains why dantrolene inhibition of single RyR channels has not been previously observed.

Background Cardiac dysfunction and arrhythmia are common and onerous cardiovascular events in end-stage renal disease (ESRD) patients, especially those on dialysis. Fibroblast growth factor (FGF)-23 is a phosphate-regulating hormone whose levels dramatically increase as renal function declines. Beyond its role in phosphorus homeostasis, FGF-23 may elicit a direct effect on the heart. Whether FGF-23 modulates ventricular cardiac rhythm is unknown, prompting us to study its role on excitation–contraction (EC) coupling. Methods We examined FGF-23 in vitro actions on EC coupling in adult rat native ventricular cardiomyocytes using patch clamp and confocal microscopy and in vivo actions on cardiac rhythm using electrocardiogram. Results Compared with vehicle treatment, FGF-23 induced a significant decrease in rat cardiomyocyte contraction, L-type Ca2+ current, systolic Ca2+ transients and sarcoplasmic reticulum (SR) load and SR Ca2+-adenosine triphosphatase 2a pump activity. FGF-23 induced pro-arrhythmogenic activity in vitro and in vivo as automatic cardiomyocyte extracontractions and premature ventricular contractions. Diastolic spontaneous Ca2+ leak (sparks and waves) was significantly increased by FGF-23 via the calmodulin kinase type II (CaMKII)-dependent pathway related to hyperphosphorylation of ryanodine receptors at the CaMKII site Ser2814. Both contraction dysfunction and spontaneous pro-arrhythmic Ca2+ events induced by FGF-23 were blocked by soluble Klotho (sKlotho). Conclusions Our results show that FGF-23 reduces contractility and enhances arrhythmogenicity through intracellular Ca2+ mishandling. Blocking its actions on the heart by improving sKlotho bioavailability may enhance cardiac function and reduce arrhythmic events frequently observed in ESRD.