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Long QT syndrome is a potentially life-threatening disease characterized by delayed repolarization of cardiomyocytes, QT interval prolongation in the electrocardiogram, and a high risk for sudden cardiac death caused by ventricular arrhythmia. The genetic type 3 of this syndrome (LQT3) is caused by gain-of-function mutations in the SCN5A cardiac sodium channel gene which mediates the fast Nav1.5 current during action potential initiation. Here, we report the analysis of LQT3 human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs). These were generated from a patient with a heterozygous p.R1644H mutation in SCN5A known to interfere with fast channel inactivation. LQT3 hiPSC-CMs recapitulated pathognomonic electrophysiological features of the disease, such as an accelerated recovery from inactivation of sodium currents as well as action potential prolongation, especially at low stimulation rates. In addition, unlike previously described LQT3 hiPSC models, we observed a high incidence of early after depolarizations (EADs) which is a trigger mechanism for arrhythmia in LQT3. Administration of specific sodium channel inhibitors was found to shorten action and field potential durations specifically in LQT3 hiPSC-CMs and antagonized EADs in a dose-dependent manner. These findings were in full agreement with the pharmacological response profile of the underlying patient and of other patients from the same family. Thus, our data demonstrate the utility of patient-specific LQT3 hiPSCs for assessing pharmacological responses to putative drugs and for improving treatment efficacies. Electronic supplementary material The online version of this article (doi:10.1007/s00395-016-0530-0) contains supplementary material, which is available to authorized users.

The cardiac intercalated disc harbors mechanical and electrical junctions as well as ion channel complexes mediating propagation of electrical impulses. Cardiac connexin43 (Cx43) co-localizes and interacts with several of the proteins located at intercalated discs in the ventricular myocardium. We have generated conditional Cx43D378stop mice lacking the last five C-terminal amino acid residues, representing a binding motif for zonula occludens protein-1 (ZO-1), and investigated the functional consequences of this mutation on cardiac physiology and morphology. Newborn and adult homozygous Cx43D378stop mice displayed markedly impaired and heterogeneous cardiac electrical activation properties and died from severe ventricular arrhythmias. Cx43 and ZO-1 were co-localized at intercalated discs in Cx43D378stop hearts, and the Cx43D378stop gap junction channels showed normal coupling properties. Patch clamp analyses of isolated adult Cx43D378stop cardiomyocytes revealed a significant decrease in sodium and potassium current densities. Furthermore, we also observed a significant loss of Nav1.5 protein from intercalated discs in Cx43D378stop hearts. The phenotypic lethality of the Cx43D378stop mutation was very similar to the one previously reported for adult Cx43 deficient (Cx43KO) mice. Yet, in contrast to Cx43KO mice, the Cx43 gap junction channel was still functional in the Cx43D378stop mutant. We conclude that the lethality of Cx43D378stop mice is independent of the loss of gap junctional intercellular communication, but most likely results from impaired cardiac sodium and potassium currents. The Cx43D378stop mice reveal for the first time that Cx43 dependent arrhythmias can develop by mechanisms other than impairment of gap junction channel function.

L-type Ca2+ current (I CaL) is a key regulatory and functional element during early embryonic cardiomyogenesis. As the embryonic heart underlies hormonal modulation, e.g. catecholamines, we aimed at studying effects of β-adrenergic stimulation on I CaL densities and inactivation kinetics during murine heart development. I CaL was recorded applying the whole-cell patch-clamp technique in ventricular myocytes of early embryonic (EDS, E9.5–11.5) and late developmental, fetal (LDS, E16.5–18.5) stages as well as adult cardiomyocytes. To distinguish between Ca2+-(CDI) and voltage-dependent inactivation (VDI), Ca2+ was replaced with Ba2+ in the extracellular recording solution. The β-adrenergic signaling pathway was simulated by applying isoproterenol (Iso). Basal current densities showed an increase of I CaL during development (EDS: −9.61 ± 1.97 pA/pF, n = 9; LDS: −13.2 ± 4.26 pA/pF, n = 12; adult: −16.1 ± 4.63 pA/pF, n = 5). Iso (1 µM) enhanced I CaL density with low effects at EDS (17.1 ± 3.58%, n = 8, P > 0.05) but strong effects at LDS (74.1 ± 3.77%, n = 8, P