Sami F. Noujaim

Arrhythmogenic Mechanisms in a Mouse Model of Catecholaminergic Polymorphic Ventricular Tachycardia

Catecholaminergic polymorphic ventricular tachycardia (VT) is a lethal familial disease characterized by bidirectional VT, polymorphic VT, and ventricular fibrillation. Catecholaminergic polymorphic VT is caused by enhanced Ca2+ release through defective ryanodine receptor (RyR2) channels. We used epicardial and endocardial optical mapping, chemical subendocardial ablation with Lugols solution, and patch clamping in a knockin (RyR2/RyR2R4496C) mouse model to investigate the arrhythmogenic mechanisms in catecholaminergic polymorphic VT. In isolated hearts, spontaneous ventricular arrhythmias occurred in 54% of 13 RyR2/RyR2R4496C and in 9% of 11 wild-type (P=0.03) littermates perfused with Ca2+and isoproterenol; 66% of 12 RyR2/RyR2R4496C and 20% of 10 wild-type hearts perfused with caffeine and epinephrine showed arrhythmias (P=0.04). Epicardial mapping showed that monomorphic VT, bidirectional VT, and polymorphic VT manifested as concentric epicardial breakthrough patterns, suggesting a focal origin in the His–Purkinje networks of either or both ventricles. Monomorphic VT was clearly unifocal, whereas bidirectional VT was bifocal. Polymorphic VT was initially multifocal but eventually became reentrant and degenerated into ventricular fibrillation. Endocardial mapping confirmed the Purkinje fiber origin of the focal arrhythmias. Chemical ablation of the right ventricular endocardial cavity with Lugols solution induced complete right bundle branch block and converted the bidirectional VT into monomorphic VT in 4 anesthetized RyR2/RyR2R4496C mice. Under current clamp, single Purkinje cells from RyR2/RyR2R4496C mouse hearts generated delayed afterdepolarization–induced triggered activity at lower frequencies and level of adrenergic stimulation than wild-type. Overall, the data demonstrate that the His–Purkinje system is an important source of focal arrhythmias in catecholaminergic polymorphic VT.

Up‐regulation of the inward rectifier K+ current (IK1) in the mouse heart accelerates and stabilizes rotors

Previous studies have suggested an important role for the inward rectifier K+ current (IK1) in stabilizing rotors responsible for ventricular tachycardia (VT) and fibrillation (VF). To test this hypothesis, we used a line of transgenic mice (TG) overexpressing Kir 2.1–green fluorescent protein (GFP) fusion protein in a cardiac‐specific manner. Optical mapping of the epicardial surface in ventricles showed that the Langendorff‐perfused TG hearts were able to sustain stable VT/VF for 350 ± 1181 s at a very high dominant frequency (DF) of 44.6 ± 4.3 Hz. In contrast, tachyarrhythmias in wild‐type hearts (WT) were short‐lived (3 ± 9 s), and the DF was 26.3 ± 5.2 Hz. The stable, high frequency, reentrant activity in TG hearts slowed down, and eventually terminated in the presence of 10 μm Ba2+, suggesting an important role for IK1. Moreover, by increasing IK1 density in a two‐dimensional computer model having realistic mouse ionic and action potential properties, a highly stable, fast rotor (≈45 Hz) could be induced. Simulations suggested that the TG hearts allowed such a fast and stable rotor because of both greater outward conductance at the core and shortened action potential duration in the core vicinity, as well as increased excitability, in part due to faster recovery of Na+ current. The latter resulted in a larger rate of increase in the local conduction velocity as a function of the distance from the core in TG compared to WT hearts, in both simulations and experiments. Finally, simulations showed that rotor frequencies were more sensitive to changes (doubling) in IK1, compared to other K+ currents. In combination, these results provide the first direct evidence that IK1 up‐regulation in the mouse heart is a substrate for stable and very fast rotors.

KCNJ2 mutation in short QT syndrome 3 results in atrial fibrillation and ventricular proarrhythmia

We describe a mutation (E299V) in KCNJ2, the gene that encodes the strong inward rectifier K+ channel protein (Kir2.1), in an 11-y-old boy. The unique short QT syndrome type-3 phenotype is associated with an extremely abbreviated QT interval (200 ms) on ECG and paroxysmal atrial fibrillation. Genetic screening identified an A896T substitution in a highly conserved region of KCNJ2 that resulted in a de novo mutation E299V. Whole-cell patch-clamp experiments showed that E299V presents an abnormally large outward IK1 at potentials above −55 mV (P < 0.001 versus wild type) due to a lack of inward rectification. Coexpression of wild-type and mutant channels to mimic the heterozygous condition still resulted in a large outward current. Coimmunoprecipitation and kinetic analysis showed that E299V and wild-type isoforms may heteromerize and that their interaction impairs function. The homomeric assembly of E299V mutant proteins actually results in gain of function. Computer simulations of ventricular excitation and propagation using both the homozygous and heterozygous conditions at three different levels of integration (single cell, 2D, and 3D) accurately reproduced the electrocardiographic phenotype of the proband, including an exceedingly short QT interval with merging of the QRS and the T wave, absence of ST segment, and peaked T waves. Numerical experiments predict that, in addition to the short QT interval, absence of inward rectification in the E299V mutation should result in atrial fibrillation. In addition, as predicted by simulations using a geometrically accurate three-dimensional ventricular model that included the His–Purkinje network, a slight reduction in ventricular excitability via 20% reduction of the sodium current should increase vulnerability to life-threatening ventricular tachyarrhythmia.

Loss of H3K4 methylation destabilizes gene expression patterns and physiological functions in adult murine cardiomyocytes

Histone H3 lysine 4 (H3K4me) methyltransferases and their cofactors are essential for embryonic development and the establishment of gene expression patterns in a cell-specific and heritable manner. However, the importance of such epigenetic marks in maintaining gene expression in adults and in initiating human disease is unclear. Here, we addressed this question using a mouse model in which we could inducibly ablate PAX interacting (with transcription-activation domain) protein 1 (PTIP), a key component of the H3K4me complex, in cardiac cells. Reducing H3K4me3 marks in differentiated cardiomyocytes was sufficient to alter gene expression profiles. One gene regulated by H3K4me3 was Kv channel-interacting protein 2 (Kcnip2), which regulates a cardiac repolarization current that is downregulated in heart failure and functions in arrhythmogenesis. This regulation led to a decreased sodium current and action potential upstroke velocity and significantly prolonged action potential duration (APD). The prolonged APD augmented intracellular calcium and in vivo systolic heart function. Treatment with isoproterenol and caffeine in this mouse model resulted in the generation of premature ventricular beats, a harbinger of lethal ventricular arrhythmias. These results suggest that the maintenance of H3K4me3 marks is necessary for the stability of a transcriptional program in differentiated cells and point to an essential function for H3K4me3 epigenetic marks in cellular homeostasis.

From Mouse to Whale A Universal Scaling Relation for the PR Interval of the Electrocardiogram of Mammals

Background—On the ECG, the PR interval measures the time taken by an electrical impulse generated in the sinoatrial node to propagate from atria to ventricles. From mouse to whale, the PR interval increases ≈101, whereas body mass (BM) augments ≈106. Scaling of many biological processes (eg, metabolic rate, life span, aortic diameter) is described by the allometric equation Y=Y0 · BMb, where Y is the biological process and b is the scaling exponent that is an integer multiple of 1/4. Hierarchical branching networks have been proposed to be the underlying mechanism for the 1/4 power allometric law. Methods and Results—We first derived analytically the allometric equation for the PR interval. We assumed that the heart behaves as a set of “fractal-like” networks that tend to minimize propagation time across the conducting system while ensuring a hemodynamically optimal atrioventricular activation sequence. Our derivation yielded the relationship PR∝BM1/4. We subsequently obtained previously published values of PR interval, heart rate, and BM of 541 mammals representing 33 species. Double-logarithmic analysis demonstrates that PR interval increases as heart rate decreases, and both variables relate to BM following the 1/4 power law. Most important, the best fit for PR versus BM is described by the equation PR=53 · BM0.24. Hence, the empirically determined exponent (0.24) is close to 1/4, as predicted. Conclusions—We have demonstrated that the PR interval of mammals scales as the 1/4 power of the BM, following the universal law for allometric scaling to ensure an optimal atrioventricular activation sequence.

Immunohistochemical characterization of the intrinsic cardiac neural plexus in whole-mount mouse heart preparations

BACKGROUND The intrinsic neural plexus of the mouse heart has not been adequately investigated despite the extensive use of this species in experimental cardiology. OBJECTIVE The purpose of this study was to determine the distribution of cholinergic, adrenergic, and sensory neural components in whole-mount mouse heart preparations using double immunohistochemical labeling. METHODS/RESULTS Intrinsic neurons were concentrated within 19 ± 3 ganglia (n = 20 mice) of varying size, scattered on the medial side of the inferior caval (caudal) vein on the right atrium and close to the pulmonary veins on the left atrium. Of a total of 1,082 ± 160 neurons, most somata (83%) were choline acetyltransferase (ChAT) immunoreactive, whereas 4% were tyrosine hydroxylase (TH) immunoreactive; 14% of ganglionic cells were biphenotypic for ChAT and TH. The most intense ChAT staining was observed in axonal varicosities. ChAT was evident in nerve fibers interconnecting intrinsic ganglia. Both ChAT and TH immunoreactivity were abundant within the nerves accessing the heart. However, epicardial TH-immunoreactive nerve fibers were predominant on the dorsal and ventral left atrium, whereas most ChAT-positive axons proceeded on the heart base toward the large intrinsic ganglia and on the epicardium of the root of the right cranial vein. Substance P-positive and calcitonin gene-related peptide-immunoreactive nerve fibers were abundant on the epicardium and within ganglia adjacent to the heart hilum. Small intensely fluorescent cells were grouped into clusters of 3 to 8 and were dispersed within large ganglia or separately on the atrial and ventricular walls. CONCLUSION Although some nerves and neuronal bundles of the mouse epicardial plexus are mixed, most express either adrenergic or cholinergic markers. Therefore, selective stimulation and/or ablation of the functionally distinct intrinsic neural pathways should allow the study of specific effects on cardiac function.

Specific residues of the cytoplasmic domains of cardiac inward rectifier potassium channels are effective antifibrillatory targets

Atrial and ventricular tachyarrhythmias can be perpetuated by up‐regulation of inward rectifier potassium channels. Thus, it may be beneficial to block inward rectifier channels under conditions in which their function becomes arrhythmogenic (e.g., inherited gain‐of‐function mutation channelopathies, ischemia, and chronic and vagally mediated atrial fibrillation). We hypothesize that the antimalarial quinoline chloroquine exerts potent antiarrhythmic effects by interacting with the cytoplasmic domains of Kir2.1 (IK1), Kir3.1 (IKACh), or Kir6.2 (IKATP) and reducing inward rectifier potassium currents. In isolated hearts of three different mammalian species, intracoronary chloroquine perfusion reduced fibrillatory frequency (atrial or ventricular), and effectively terminated the arrhythmia with resumption of sinus rhythm. In patch‐clamp experiments chloroquine blocked IK1, IKACh, and IKATP. Comparative molecular modeling and ligand docking of chloroquine in the intracellular domains of Kir2.1, Kir3.1, and Kir6.2 suggested that chloroquine blocks or reduces potassium flow by interacting with negatively charged amino acids facing the ion permeation vestibule of the channel in question. These results open a novel path toward discovering antiarrhythmic pharmacophores that target specific residues of the cytoplasmic domain of inward rectifier potassium channels.—Noujaim, S. F., Stuckey, J. A., Ponce‐Balbuena, D., Ferrer‐Villada, T., López‐Izquierdo, A., Pandit, S., Calvo, C. J., Grzeda, K. R., Berenfeld, O., Sánchez Chapula, J. A., Jalife, J. Specific residues of the cytoplasmic domains of cardiac inward rectifier potassium channels are effective antifibrillatory targets. FASEB J. 24, 4302–4312 (2010). www.fasebj.org

A null mutation of the neuronal sodium channel NaV1.6 disrupts action potential propagation and excitation-contraction coupling in the mouse heart

Evidence supports the expression of brain‐type sodium channels in the heart. Their functional role, however, remains controversial. We used global NaV1.6‐null mice to test the hypothesis that NaV1.6 contributes to the maintenance of propagation in the myocardium and to excitation‐contraction (EC) coupling. We demonstrated expression of transcripts encoding full‐length NaV1.6 in isolated ventricular myocytes and confirmed the striated pattern of NaV1.6 fluorescence in myocytes. On the ECG, the PR and QRS intervals were prolonged in the null mice, and the Ca2+ transients were longer in the null cells. Under patch clamping, at holding potential (HP) = –120 mV, the peak INa was similar in both phenotypes. However, at HP = –70 mV, the peak INa was smaller in the nulls. In optical mapping, at 4 mM [K+]o, 17 null hearts showed slight (7%) reduction of ventricular conduction velocity (CV) compared to 16 wild‐type hearts. At 12 mM [K+]o, CV was 25% slower in a subset of 9 null vs. 9 wild‐type hearts. These results highlight the importance of neuronal sodium channels in the heart, whereby NaV1.6 participates in EC coupling, and represents an intrinsic depolarizing reserve that contributes to excitation.—Noujaim, S. F., Kaur, K., Milstein, M., Jones, J. M., Furspan, P., Jiang, D., Auerbach, D. S., Herron, T., Meisler, M. H., Jalife, J. A null mutation of the neuronal sodium channel NaV1.6 disrupts action potential propagation and excitation‐contraction coupling in the mouse heart. FASEB J. 26, 63–72 (2012). www.fasebj.org

Universal scaling law of electrical turbulence in the mammalian heart

Many biological processes, such as metabolic rate and life span, scale with body mass (BM) according to the universal law of allometric scaling: Y = aBMb (Y, biological process; b, scaling exponent). We investigated whether the temporal properties of ventricular fibrillation (VF), the major cause of sudden and unexpected cardiac death, scale with BM. By using high-resolution optical mapping, numerical simulations and metaanalysis of VF data in 11 mammalian species, we demonstrate that the interbeat interval of VF scales as VFcycle length = 53 × BM1/4, spanning more than four orders of magnitude in BM from mouse to horse.

Structural bases for the different anti-fibrillatory effects of chloroquine and quinidine

AIMS Chloroquine, an anti-malarial quinoline, is structurally similar to quinidine. Both drugs have been shown to block ion channels. We tested the hypothesis that chloroquines mode of interaction with the vestibule of the cytoplasmic domain of the inward rectifier potassium channel Kir2.1 makes it a more effective I(K1) blocker and anti-fibrillatory agent than quinidine. METHODS AND RESULTS We used comparative molecular modelling and ligand docking of the three-dimensional structures of quinidine and chloroquine in the intracellular domain of Kir2.1. Simulations predicted that chloroquine effectively blocks potassium flow by binding at the centre of the ion permeation vestibule of Kir2.1. In contrast, quinidine binds the vestibular side, only partially blocking ion movement. We tested the modelling predictions in Kir2.1-expressing human embryonic kidney (HEK)-293 cells. The half-maximal inhibitory concentration for chloroquine block of I(K1) was 1.2 µM, while that of quinidine was 57 µM. Finally, we used optical mapping of Langendorff-perfused mouse hearts with cardiac-specific Kir2.1 up-regulation to compare the anti-fibrillatory effects of the drugs. In five of six hearts, 10 μM quinidine slowed the frequency but did not terminate the tachyarrhythmia. In five of five hearts, 10 μM chloroquine terminated the arrhythmia, restoring sinus rhythm. CONCLUSION Quinidine only partially blocks I(K1). Chloroquine binds at the centre of the ion permeation vestibule of Kir2.1, which makes it a more effective I(K1) blocker and anti-fibrillatory agent than quinidine. Integrating the structural biology of drug-ion channel interactions with cellular electrophysiology and optical mapping is an excellent approach to understand the molecular mechanisms of anti-arrhythmic drug action and for drug discovery.