No abstract

Long QT syndrome (LQTS) is a disorder of ventricular repolarization that predisposes affected individuals to lethal cardiac arrhythmias. To date, an appropriate animal model of inherited LQTS does not exist. The zebrafish is a powerful vertebrate model used to dissect molecular pathways of cardiovascular development and disease. Because fundamental electrical properties of the zebrafish heart are remarkably similar to those of the human heart, the zebrafish may be an appropriate model for studying human inherited arrhythmias. Here we describe the molecular, cellular, and electrophysiological basis of a zebrafish mutant characterized by ventricular asystole. Genetic mapping and direct sequencing identify the affected gene as kcnh2, which encodes the channel responsible for the rapidly activating delayed rectifier K ؉ current (IKr). We show that complete loss of functional IKr in embryonic hearts leads to ventricular cell membrane depolarization, inability to generate action potentials (APs), and disrupted calcium release. A small hyperpolarizing current restores spontaneous APs, implying wildtype function of other ionic currents critical for AP generation. Heterozygous fish manifest overt cellular and electrocardiographic evidence for delayed ventricular repolarization. Our findings provide insight into the pathogenesis of homozygous kcnh2 mutations and expand the use of zebrafish mutants as a model system to study human arrhythmias.cardiac ͉ development ͉ kcnh2 ͉ arrhythmia ͉ electrophysiology A utosomal dominant KCNH2 mutations account for Ϸ45% of mutation-positive long QT syndrome (LQTS) (1, 2). Recessive KCNH2 mutations are rarely reported, implying that most homozygous mutations are embryonic-lethal (3, 4). The cellular mechanisms underlying embryonic lethality are not known. Although LQTS has been extensively characterized in humans, establishing an animal model would be helpful to identify genes that modify phenotypic expressivity or to screen against compounds that cause acquired forms of LQTS. To date, an appropriate animal model of inherited LQTS does not exist. Transgenic strategies and targeted deletion of genes that regulate rapidly activating delayed rectifier K ϩ current (I Kr ) and slow delayed rectifier K ϩ current (I Ks ) in mice fail to produce a significant phenotype (5-7). The mouse heart rate is nearly 10 times faster than that of humans, and thus a distinct cadre of ion channels facilitates ventricular repolarization in mice (5-8).Although the zebrafish heart is two-chambered, its fundamental electrical properties are remarkably similar to those of humans (9). For example, embryonic and adult heart rates are similar to those of humans (10, 11), and the relationship between QT interval and heart rate parallels that of humans (11). Forward-genetic screens using zebrafish have identified many cardiovascular mutants for analysis (12, 13), thereby revealing critical pathways in cardiovascular development that parallel those of higher vertebrates (14). Large clutch size facilitates positional clo...

Our results reveal a molecular mechanism by which lipid overload-induced mitochondrial ROS generation causes mitochondrial dysfunction by inducing post-translational modifications of mitochondrial proteins that regulate mitochondrial dynamics. These findings provide a novel mechanism for mitochondrial dysfunction in lipotoxic cardiomyopathy.

Single ventricular myocytes paced at a constant rate and held at a constant temperature exhibit beat-to-beat variations in action potential duration (APD). In this study we sought to quantify this variability, assess its mechanism, and determine its responsiveness to electrotonic interactions with another myocyte. Interbeat APD(90) (90% repolarization) of single cells was normally distributed. We thus quantified APD(90) variability as the coefficient of variability, CV = (SD/mean APD(90)) x 100. The mean +/- SD of the CV in normal solution was 2.3 +/- 0.9 (132 cells). Extracellular TTX (13 microM) and intracellular EGTA (14 mM) both significantly reduced the CV by 44 and 26%, respectively. When applied in combination the CV fell by 54%. In contrast, inhibition of the rapid delayed rectifier current with L-691,121 (100 nM) increased the CV by 300%. The CV was also significantly reduced by 35% when two normal myocytes were electrically connected with a junctional resistance (R(j)) of 100 MOmega. Electrical coupling (R(j) = 100 MOmega) of a normal myocyte to one producing early afterdepolarization (EAD) completely blocked EAD formation. These results indicate that beat-to-beat APD variability is likely mediated by stochastic behavior of ion channels and that electrotonic interactions act to limit temporal dispersion of refractoriness, a major contributor to arrhythmogenesis.