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Aims Diseased atria are characterized by functional and structural heterogeneities, adding to abnormal impulse generation and propagation. These heterogeneities are thought to lie at the origin of fractionated electrograms recorded during sinus rhythm (SR) in atrial fibrillation (AF) patients and are assumed to be involved in the onset and perpetuation (e.g. by re-entry) of this disorder. The underlying mechanisms, however, remain incompletely understood. Here, we tested whether regions of dense fibrosis could create an electrically isolated conduction pathway (EICP) in which re-entry could be established via ectopy and local block to become 'trapped'. We also investigated whether this could generate local fractionated electrograms and whether the re-entrant wave could 'escape' and cause a global tachyarrhythmia due to dynamic changes at a connecting isthmus. Methods and results To precisely control and explore the geometrical properties of EICPs, we used light-gated depolarizing ion channels and patterned illumination for creating specific non-conducting regions in silico and in vitro. Insight from these studies was used for complementary investigations in virtual human atria with localized fibrosis. We demonstrated that a re-entrant tachyarrhythmia can exist locally within an EICP with SR prevailing in the surrounding tissue and identified conditions under which re-entry could escape from the EICP, thereby converting a local latent arrhythmic source into an active driver with global impact on the heart. In a realistic three-dimensional model of human atria, unipolar epicardial pseudo-electrograms showed fractionation at the site of 'trapped re-entry' in coexistence with regular SR electrograms elsewhere in the atria. Upon escape of the re-entrant wave, acute arrhythmia onset was observed. Conclusions Trapped re-entry as a latent source of arrhythmogenesis can explain the sudden onset of focal arrhythmias, which are able to transgress into AF. Our study might help to improve the effectiveness of ablation of aberrant cardiac electrical signals in clinical practice. [ABSTRACT FROM AUTHOR]

Aims Human-induced pluripotent stem cell-cardiomyocytes (hiPSC-CMs) are widely used to study arrhythmia-associated mutations in ion channels. Among these, the cardiac sodium channel SCN5A undergoes foetal-to-adult isoform switching around birth. Conventional hiPSC-CM cultures, which are phenotypically foetal, have thus far been unable to capture mutations in adult gene isoforms. Here, we investigated whether tri-cellular cross-talk in a three-dimensional (3D) cardiac microtissue (MT) promoted post-natal SCN5A maturation in hiPSC-CMs. Methods and results We derived patient hiPSC-CMs carrying compound mutations in the adult SCN5A exon 6B and exon 4. Electrophysiological properties of patient hiPSC-CMs in monolayer were not altered by the exon 6B mutation compared with isogenic controls since it is not expressed; further, CRISPR/Cas9-mediated excision of the foetal exon 6A did not promote adult SCN5A expression. However, when hiPSC-CMs were matured in 3D cardiac MTs, SCN5A underwent isoform switch and the functional consequences of the mutation located in exon 6B were revealed. Up-regulation of the splicing factor muscleblind-like protein 1 (MBNL1) drove SCN5A post-natal maturation in microtissues since its overexpression in hiPSC-CMs was sufficient to promote exon 6B inclusion, whilst knocking-out MBNL1 failed to foster isoform switch. Conclusions Our study shows that (i) the tri-cellular cardiac microtissues promote post-natal SCN5A isoform switch in hiPSC-CMs, (ii) adult splicing of SCN5A is driven by MBNL1 in these tissues, and (iii) this model can be used for examining post-natal cardiac arrhythmias due to mutations in the exon 6B. Translational perspective The cardiac sodium channel is essential for conducting the electrical impulse in the heart. Postnatal alternative splicing regulation causes mutual exclusive inclusion of fetal or adult exons of the corresponding gene, SCN5A. Typically, immature hiPSCCMs fall short in studying the effect of mutations located in the adult exon. We describe here that an innovative tri-cellular three-dimensional cardiac microtissue culture promotes hiPSC-CMs maturation through upregulation of MBNL1, thus revealing the effect of a pathogenic genetic variant located in the SCN5A adult exon. These results help advancing the use of hiPSC-CMs in studying adult heart disease and for developing personalized medicine applications. [ABSTRACT FROM AUTHOR]

Aims Cardiac electrical activity is extraordinarily robust. However, when it goes wrong it can have fatal consequences. Electrical activity in the heart is controlled by the carefully orchestrated activity of more than a dozen different ion conductances. While there is considerable variability in cardiac ion channel expression levels between individuals, studies in rodents have indicated that there are modules of ion channels whose expression co-vary. The aim of this study was to investigate whether meta-analytic co-expression analysis of large-scale gene expression datasets could identify modules of co-expressed cardiac ion channel genes in human hearts that are of functional importance. Methods and results Meta-analysis of 3653 public human RNA-seq datasets identified a strong correlation between expression of CACNA1C (L-type calcium current, I CaL) and KCNH2 (rapid delayed rectifier K+ current, I Kr), which was also observed in human adult heart tissue samples. In silico modelling suggested that co-expression of CACNA1C and KCNH2 would limit the variability in action potential duration seen with variations in expression of ion channel genes and reduce susceptibility to early afterdepolarizations, a surrogate marker for proarrhythmia. We also found that levels of KCNH2 and CACNA1C expression are correlated in human-induced pluripotent stem cell-derived cardiac myocytes and the levels of CACNA1C and KCNH2 expression were inversely correlated with the magnitude of changes in repolarization duration following inhibition of I Kr. Conclusion Meta-analytic approaches of multiple independent human gene expression datasets can be used to identify gene modules that are important for regulating heart function. Specifically, we have verified that there is co-expression of CACNA1C and KCNH2 ion channel genes in human heart tissue, and in silico analyses suggest that CACNA1C–KCNH2 co-expression increases the robustness of cardiac electrical activity. [ABSTRACT FROM AUTHOR]

Aims Several inherited arrhythmic diseases have been linked to single gene mutations in cardiac ion channels and interacting proteins. However, the mechanisms underlying most arrhythmias, are thought to involve altered regulation of the expression of multiple effectors. In this study, we aimed to examine the role of a transcription factor (TF) belonging to the Iroquois homeobox family, IRX5, in cardiac electrical function. Methods and results Using human cardiac tissues, transcriptomic correlative analyses between IRX5 and genes involved in cardiac electrical activity showed that in human ventricular compartment, IRX5 expression strongly correlated to the expression of major actors of cardiac conduction, including the sodium channel, Nav1.5, and Connexin 40 (Cx40). We then generated human-induced pluripotent stem cells (hiPSCs) derived from two Hamamy syndrome-affected patients carrying distinct homozygous loss-of-function mutations in IRX5 gene. Cardiomyocytes derived from these hiPSCs showed impaired cardiac gene expression programme, including misregulation in the control of Nav1.5 and Cx40 expression. In accordance with the prolonged QRS interval observed in Hamamy syndrome patients, a slower ventricular action potential depolarization due to sodium current reduction was observed on electrophysiological analyses performed on patient-derived cardiomyocytes, confirming the functional role of IRX5 in electrical conduction. Finally, a cardiac TF complex was newly identified, composed by IRX5 and GATA4, in which IRX5 potentiated GATA4-induction of SCN5A expression. Conclusion Altogether, this work unveils a key role for IRX5 in the regulation of human ventricular depolarization and cardiac electrical conduction, providing therefore new insights into our understanding of cardiac diseases. [ABSTRACT FROM AUTHOR]

Aims Atrial fibrillation (AF) is the most common type of cardiac arrhythmias, whose incidence is likely to increase with the aging of the population. It is considered a progressive condition, frequently observed as a complication of other cardiovascular disorders. However, recent genetic studies revealed the presence of several mutations and variants linked to AF, findings that define AF as a multifactorial disease. Due to the complex genetics and paucity of models, molecular mechanisms underlying the initiation of AF are still poorly understood. Here we investigate the pathophysiological mechanisms of a familial form of AF, with particular attention to the identification of putative triggering cellular mechanisms, using patient's derived cardiomyocytes (CMs) differentiated from induced pluripotent stem cells (iPSCs). Methods and results Here we report the clinical case of three siblings with untreatable persistent AF whose whole-exome sequence analysis revealed several mutated genes. To understand the pathophysiology of this multifactorial form of AF we generated three iPSC clones from two of these patients and differentiated these cells towards the cardiac lineage. Electrophysiological characterization of patient-derived CMs (AF-CMs) revealed that they have higher beating rates compared to control (CTRL)-CMs. The analysis showed an increased contribution of the I f and I CaL currents. No differences were observed in the repolarizing current I Kr and in the sarcoplasmic reticulum calcium handling. Paced AF-CMs presented significantly prolonged action potentials and, under stressful conditions, generated both delayed after-depolarizations of bigger amplitude and more ectopic beats than CTRL cells. Conclusions Our results demonstrate that the common genetic background of the patients induces functional alterations of I f and I CaL currents leading to a cardiac substrate more prone to develop arrhythmias under demanding conditions. To our knowledge this is the first report that, using patient-derived CMs differentiated from iPSC, suggests a plausible cellular mechanism underlying this complex familial form of AF. [ABSTRACT FROM AUTHOR]

Aims Caveolinopathies are a family of genetic disorders arising from alterations of the caveolin-3 (cav-3) gene. The T78M cav-3 variant has been associated with both skeletal and cardiac muscle pathologies but its functional contribution, especially to cardiac diseases, is still controversial. Here, we evaluated the effect of the T78M cav-3 variant on cardiac ion channel function and membrane excitability. Methods and results We transfected either the wild type (WT) or T78M cav-3 in caveolin-1 knock-out mouse embryonic fibroblasts and found by immunofluorescence and electron microscopy that both are expressed at the plasma membrane and form caveolae. Two ion channels known to interact and co-immunoprecipitate with the cav-3, hKv1.5 and hHCN4, interact also with T78M cav-3 and reside in lipid rafts. Electrophysiological analysis showed that the T78M cav-3 causes hKv1.5 channels to activate and inactivate at more hyperpolarized potentials and the hHCN4 channels to activate at more depolarized potentials, in a dominant way. In spontaneously beating neonatal cardiomyocytes, the expression of the T78M cav-3 significantly increased action potential peak-to-peak variability without altering neither the mean rate nor the maximum diastolic potential. We also found that in a small cohort of patients with supraventricular arrhythmias, the T78M cav-3 variant is more frequent than in the general population. Finally, in silico analysis of both sinoatrial and atrial cell models confirmed that the T78M-dependent changes are compatible with a pro-arrhythmic effect. Conclusion This study demonstrates that the T78M cav-3 induces complex modifications in ion channel function that ultimately alter membrane excitability. The presence of the T78M cav-3 can thus generate a susceptible substrate that, in concert with other structural alterations and/or genetic mutations, may become arrhythmogenic. [ABSTRACT FROM AUTHOR]

Aims Atrial fibrillation (AF) is the most common cardiac arrhythmia and often involves reentrant electrical activation (e.g. spiral waves). Drug therapy for AF can have serious side effects including proarrhythmia, while electrical shock therapy is associated with discomfort and tissue damage. Hypothetically, forced expression and subsequent activation of light-gated cation channels in cardiomyocytes might deliver a depolarizing force sufficient for defibrillation, thereby circumventing the aforementioned drawbacks. We therefore investigated the feasibility of light-induced spiral wave termination through cardiac optogenetics. Methods and results Neonatal rat atrial cardiomyocyte monolayers were transduced with lentiviral vectors encoding light-activated Ca2+-translocating channelrhodopsin (CatCh; LV.CatCh∼eYFP↑) or eYFP (LV.eYFP↑) as control, and burst-paced to induce spiral waves rotating around functional cores. Effects of CatCh activation on reentry were investigated by optical and multi-electrode array (MEA) mapping. Western blot analyses and immunocytology confirmed transgene expression. Brief blue light pulses (10 ms/470 nm) triggered action potentials only in LV.CatCh∼eYFP↑-transduced cultures, confirming functional CatCh-mediated current. Prolonged light pulses (500 ms) resulted in reentry termination in 100% of LV.CatCh∼eYFP↑-transduced cultures (n = 31) vs. 0% of LV.eYFP↑-transduced cultures (n = 11). Here, CatCh activation caused uniform depolarization, thereby decreasing overall excitability (MEA peak-to-peak amplitude decreased 251.3 ± 217.1 vs. 9.2 ± 9.5 μV in controls). Consequently, functional coresize increased and phase singularities (PSs) drifted, leading to reentry termination by PS–PS or PS–boundary collisions. Conclusion This study shows that spiral waves in atrial cardiomyocyte monolayers can be terminated effectively by a light-induced depolarizing current, produced by the arrhythmogenic substrate itself, upon optogenetic engineering. These results provide proof-of-concept for shockless defibrillation. [ABSTRACT FROM PUBLISHER]

Background A recent genome-wide association study identified a susceptibility locus for atrial fibrillation at the KCNN3 gene. Since the KCNN3 gene encodes for a small conductance calcium-activated potassium channel, we hypothesized that overexpression of the SK3 channel increases susceptibility to cardiac arrhythmias. Methods and results We characterized the cardiac electrophysiological phenotype of a mouse line with overexpression of the SK3 channel. We generated homozygote (SK3T/T) and heterozygote (SK3+/T) mice with overexpression of the channel and compared them with wild-type (WT) controls. We observed a high incidence of sudden death among SK3T/T mice (7 of 19 SK3T/T mice). Ambulatory monitoring demonstrated that sudden death was due to heart block and bradyarrhythmias. SK3T/T mice displayed normal body weight, temperature, and cardiac function on echocardiography; however, histological analysis demonstrated that these mice have abnormal atrioventricular node morphology. Optical mapping demonstrated that SK3T/T mice have slower ventricular conduction compared with WT controls (SK3T/T vs. WT; 0.45 ± 0.04 vs. 0.60 ± 0.09 mm/ms, P = 0.001). Programmed stimulation in 1-month-old SK3T/T mice demonstrated inducible atrial arrhythmias (50% of SK3T/T vs. 0% of WT mice) and also a shorter atrioventricular nodal refractory period (SK3T/T vs. WT; 43 ± 6 vs. 52 ± 9 ms, P = 0.02). Three-month-old SK3T/T mice on the other hand displayed a trend towards a more prolonged atrioventricular nodal refractory period (SK3T/T vs. WT; 61 ± 1 vs. 52 ± 6 ms, P = 0.06). Conclusion Overexpression of the SK3 channel causes an increased risk of sudden death associated with bradyarrhythmias and heart block, possibly due to atrioventricular nodal dysfunction. [ABSTRACT FROM AUTHOR]

Aims Ion channel remodelling and ventricular conduction system (VCS) alterations play relevant roles in the generation of cardiac arrhythmias, but the interaction between ion channel remodelling and cardiac conduction system dysfunctions in an arrhythmogenic context remain unexplored. Methods and results We have used a transgenic mouse line previously characterized as an animal model of Long QT Syndrome (LQTS) to analyse ion channel remodelling and VCS configuration. Reverse transcriptase-PCR and immunohistochemistry analysis showed early cardiac sodium channel upregulation at embryonic stages prior to the onset of Kv potassium channel remodelling, and cardiac hypertrophy at foetal stages. In line with these findings, patch-clamp assays demonstrated changes in sodium current density and a slowing of recovery from inactivation. Functional analysis by optical mapping revealed an immature ventricular activation pattern as well as an increase in the total left ventricle activation time in foetal transgenic hearts. Morphological analysis of LQTS transgenic mice in a Cx40GFP/+background demonstrated VCS dysmorphogenesis during heart development. Conclusions Our data demonstrate early sodium channel remodelling secondary to IKs blockage in a mouse model of LQTS leading to morphological and functional anomalies in the developing VCS and cardiac hypertrophy. These results provide new insights into the mechanisms underlying foetal and neonatal cardiac electrophysiological disorders, which might help understand how molecular, functional, and morphological alterations are linked to clinical pathologies such as cardiac congenital anomalies, arrhythmias, and perinatal sudden death. [ABSTRACT FROM AUTHOR]

Aims Cardiac hypertrophy is accompanied by reprogramming of gene expression, where the altered expression of ion channels decreases electrical stability and increases the risk of life-threatening arrhythmias. However, the underlying mechanisms are not fully understood. Here, we analysed the role of the depolarizing current If which has been hypothesized to contribute to arrhythmogenesis in the hypertrophied ventricle. Methods and results We used transverse aortic constriction in mice to induce ventricular hypertrophy. This resulted in an increased number of If positive ventricular myocytes as well as a strongly enhanced and accelerated If when compared with controls. Of the four HCN (hyperpolarization-activated cyclic nucleotide-gated channels) isoforms mediating If, HCN2 and HCN4 were the predominantly expressed subunits in healthy as well as hypertrophied hearts. Unexpectedly, only the HCN1 transcript was significantly upregulated in response to hypertrophy. However, the combined deletion of HCN2 and HCN4 disrupted ventricular If completely. The lack of If in hypertrophic double-knockouts resulted in a strong attenuation of pro-arrhythmogenic parameters characteristically observed in hypertrophic hearts. In particular, prolongation of the action potential was significantly decreased and lengthening of the QT interval was reduced. Conclusions We suggest that the strongly increased HCN channel activity in hypertrophied myocytes prolongs the repolarization of the ventricular action potential and thereby may increase the arrhythmogenic potential. Our results provide for the first time a direct link between an upregulation of ventricular If and a diminished repolarization reserve in cardiac hypertrophy. [ABSTRACT FROM AUTHOR]

Aims Short-QT syndrome (SQTS) is a recently recognized disorder associated with atrial fibrillation (AF) and sudden death due to ventricular arrhythmias. Mutations in several ion channel genes have been linked to SQTS; however, the mechanism remains unclear. This study describes a novel heterozygous gain-of-function mutation in the inward rectifier potassium channel gene, KCNJ2, identified in SQTS. Methods and results We studied an 8-year-old girl with a markedly short-QT interval (QT = 172 ms, QTc = 194 ms) who suffered from paroxysmal AF. Mutational analysis identified a novel heterozygous KCNJ2 mutation, M301K. Functional assays displayed no Kir2.1 currents when M301K channels were expressed alone. However, co-expression of wild-type (WT) with M301K resulted in larger outward currents than the WT at more than −30 mV. These results suggest a gain-of-function type modulation due to decreased inward rectification. Furthermore, we analysed the functional significance of the amino acid charge at M301 (neutral) by changing the residue. As with M301K, in M301R (positive), the homozygous channels were non-functional, whereas the heterozygous channels demonstrated decreased inward rectification. Meanwhile, the currents recorded in M301A (neutral) showed normal inward rectification under both homo- and heterozygous conditions. Heterozygous overexpression of WT and M301K in neonatal rat ventricular myocytes exhibited markedly shorter action potential durations than the WT alone. Conclusion In this study, we identified a novel KCNJ2 gain-of-function mutation, M301K, associated with SQTS. Functional assays revealed no functional currents in the homozygous channels, whereas impaired inward rectification demonstrated under the heterozygous condition resulted in larger outward currents, which is a novel mechanism predisposing SQTS. [ABSTRACT FROM AUTHOR]

Aims Epidermal growth factor receptor (EGFR) plays a critical role in the development and function of the heart. Previous studies have demonstrated that EGFR is involved in regulating electrical excitability of the heart. The present study was designed to investigate whether EGFR activation would mediate cardiac arrhythmias induced by reperfusion in anaesthetized rats. Methods and results Reperfusion arrhythmias were induced by 10 min ligation of the left anterior descending coronary artery, followed by a 30 min reperfusion in anaesthetized rats. The incidence and severity of cardiac arrhythmias were significantly reduced by pre-treatment with the EGFR kinase inhibitor AG556. The phosphorylation level of myocardial EGFR was increased during ischaemia and at early reperfusion. Intramyocardial transfection of EGFR siRNA reduced EGFR mRNA and protein, and decreased the incidence of ventricular fibrillation induced by reperfusion. Interestingly, tyrosine phosphorylation levels of cardiac Na+ channels (INa) and L-type Ca2+ channels (ICa,L) were significantly increased at time points corresponding to the alteration of EGFR phosphorylation levels during reperfusion. AG556 pre-treatment countered the increased tyrosine phosphorylation level of Na+ and L-type Ca2+ channels induced by reperfusion. Patch-clamp studies proved that AG556 could inhibit INa and ICa,L in rat ventricular myocytes. No significant alteration was observed in tyrosine phosphorylation levels of cardiac Kv4.2 and Kir2.1 channels during reperfusion. Conclusion These results demonstrate for the first time that EGFR plays an important role in the genesis of arrhythmias induced by reperfusion, which is likely mediated at least in part by enhancing tyrosine phosphorylation of cardiac Na+ and L-type Ca2+ channels. [ABSTRACT FROM AUTHOR]

Aims It is well established that dysfunction of voltage-dependent ion channels results in arrhythmias and conduction disturbances in the foetal and adult heart. However, the involvement of voltage-insensitive cationic TRPC (transient receptor potential canonical) channels remains unclear. We assessed the hypothesis that TRPC channels play a crucial role in the spontaneous activity of the developing heart. Methods and results TRPC isoforms were investigated in isolated hearts obtained from 4-day-old chick embryos. Using RT-PCR, western blotting and co-immunoprecipitation, we report for the first time that TRPC1, 3, 4, 5, 6, and 7 isoforms are expressed at the mRNA and protein levels and that they can form a macromolecular complex with the α1C subunit of the L-type voltage-gated calcium channel (Cav1.2) in atria and ventricle. Using ex vivo electrocardiograms, electrograms of isolated atria and ventricle and ventricular mechanograms, we found that inhibition of TRPC channels by SKF-96365 leads to negative chrono-, dromo-, and inotropic effects, prolongs the QT interval, and provokes first- and second-degree atrioventricular blocks. Pyr3, a specific antagonist of TRPC3, affected essentially atrioventricular conduction. On the other hand, specific blockade of the L-type calcium channel with nifedipine rapidly stopped ventricular contractile activity without affecting rhythmic electrical activity. Conclusions These results give new insights into the key role that TRPC channels, via interaction with the Cav1.2 channel, play in regulation of cardiac pacemaking, conduction, ventricular activity, and contractility during cardiogenesis. [ABSTRACT FROM PUBLISHER]

The transcriptional regulation orchestrating the development of the heart is increasingly recognized to play an essential role in the regulation of ion channel and gap junction gene expression and consequently the proper generation and conduction of the cardiac electrical impulse. This has led to the realization that in some instances, abnormal cardiac electrical function and arrhythmias in the postnatal heart may stem from a developmental abnormality causing maintained (epigenetic) changes in gene regulation. The role of developmental genes in the regulation of cardiac electrical function is further underscored by recent genome-wide association studies that provide strong evidence that common genetic variation, at loci harbouring these genes, modulates electrocardiographic indices of conduction and repolarization and susceptibility to arrhythmia. Here we discuss recent findings and provide background insight into these complex mechanisms. [ABSTRACT FROM AUTHOR]

Aims The level of inhibition of the human Ether-à-go-go-related gene (hERG) channel is one of the earliest preclinical markers used to predict the risk of a compound causing Torsade-de-Pointes (TdP) arrhythmias. While avoiding the use of drugs with maximum therapeutic concentrations within 30-fold of their hERG inhibitory concentration 50% (IC50) values has been suggested, there are drugs that are exceptions to this rule: hERG inhibitors that do not cause TdP, and drugs that can cause TdP but are not strong hERG inhibitors. In this study, we investigate whether a simulated evaluation of multi-channel effects could be used to improve this early prediction of TdP risk. Methods and results We collected multiple ion channel data (hERG, Na, l-type Ca) on 31 drugs associated with varied risks of TdP. To integrate the information on multi-channel block, we have performed simulations with a variety of mathematical models of cardiac cells (for rabbit, dog, and human ventricular myocyte models). Drug action is modelled using IC50 values, and therapeutic drug concentrations to calculate the proportion of blocked channels and the channel conductances are modified accordingly. Various pacing protocols are simulated, and classification analysis is performed to evaluate the predictive power of the models for TdP risk. We find that simulation of action potential duration prolongation, at therapeutic concentrations, provides improved prediction of the TdP risk associated with a compound, above that provided by existing markers. Conclusion The suggested calculations improve the reliability of early cardiac safety assessments, beyond those based solely on a hERG block effect. [ABSTRACT FROM PUBLISHER]

Nitric oxide (NO) synthetized by essentially all cardiac cell types plays a key role in the regulation of cardiac function. Recent evidence shows that NO modulates the activity of cardiac ion channels implicated in the genesis of the cardiac action potential and exerts anti-arrhythmic properties under some circumstances. We review the effects of NO on cardiac ion channels and the signalling pathways, including cGMP-dependent (protein kinase G and cGMP-regulated phosphodiesterases) and cGMP-independent mechanisms (S-nitrosylation and direct effects on G proteins) and finally the role of NO in the genesis of cardiac arrhythmias during ischemia–reperfusion, heart failure, long QT syndrome, atrial fibrillation, and sudden cardiac death. [ABSTRACT FROM PUBLISHER]

Aims: The effect of histone deacetylase inhibitors on dystrophic heart function is not established. To investigate this aspect, dystrophic mdx mice and wild-type (WT) animals were treated 90 days either with suberoylanilide hydroxamic acid (SAHA, 5 mg/kg/day) or with an equivalent amount of vehicle. [ABSTRACT FROM PUBLISHER]

Aims: The present study was designed to investigate whether the beneficial effects of β-blocker propranolol are related to regulation of microRNA miR-1. [ABSTRACT FROM PUBLISHER]

Aims: The pathological proliferation of cardiac fibroblasts (CFs) in response to heart injury results in fibrosis, which correlates with arrhythmia generation and heart failure. Here we systematically examined the effect of fibroblast-derived paracrine factors on electrical propagation in cardiomyocytes. [ABSTRACT FROM PUBLISHER]

Cardiovascular disease is the leading cause of morbidity and mortality in developed countries. The pathological process of the heart is associated with an altered expression profile of genes that are important for cardiac function. MicroRNAs (miRNAs) have recently emerged as one of the central players of gene expression regulation. The implications of miRNAs in the pathological process of the cardiovascular system have recently been recognized, and research on miRNAs in relation to cardiovascular disease has now become a most rapidly evolving field. In this review, we focus on miRNAs and control of cardiac excitability, aiming to provide a comprehensive overview on the available experimental data on regulation of cardiac conduction, repolarization, and automaticity by miRNAs. Aberrant expression of miRNAs in the diseased state of the heart and their arrhythmogenic or anti-arrhythmic potential will be discussed. Finally, the innovative miRNA-interference technologies developed lately for manipulating the action of miRNAs by interfering with their expression, stability, and function as new approaches for miRNA research and gene therapy will be introduced. [ABSTRACT FROM PUBLISHER]

Aims: Impaired repolarization in cardiac myocytes can lead to long QT syndrome (LQTS), with delayed repolarization and increased susceptibility to Torsades de Pointes (TdP) arrhythmias. Current pharmacological treatment of LQTS is often inadequate. This study sought to evaluate the antiarrhythmic effect of a novel compound (NS1643) that activates the rapid delayed-rectifier K+ current, IKr, in two rabbit models of acquired LQTS. [ABSTRACT FROM PUBLISHER]

Aim: Clinical use of amiodarone is associated with occasional development of torsade de pointes (TdP). However, preclinical models have failed to demonstrate the proarrhythmic potential of amiodarone. The objective of this study was to reveal and explain the pro- and anti-arrhythmic effects of acute exposure to amiodarone in an animal model. [ABSTRACT FROM PUBLISHER]

In the cardiac dyad, sarcolemmal L-type Ca2+ channels (LCCs) and sarcoplasmic reticulum (SR) Ca2+ release channels (RyR) are structurally in close proximity. This organization provides for an efficient functional coupling, tuning SR Ca2+ release for optimal contraction of the myocyte. Given that LCC are regulated by the prevailing [Ca2+], this structural organization is the setting for feedback mechanisms and crosstalk. A defective coupling of Ca2+ influx via LCC to activation of RyR has been implicated in reduced SR Ca2+ release in heart failure. Both functional changes in LCC properties and structural re-organization of LCC in T-tubules could be involved. LCC are regulated by cytosolic Ca2+, and crosstalk with SR Ca2+ handling occurs on a long-term basis, i.e. during steady-state changes in heart rate, on an intermediate-term basis, i.e. on a beat-to-beat basis during sudden rate changes, and on a very short- or immediate-term basis, i.e. during a single heartbeat. We review the properties and consequences of these different feedback mechanisms and the changes in heart failure and cardiac hypertrophy that have thus far been studied. [ABSTRACT FROM PUBLISHER]

Abstract: Objectives: Patients with a missense mutation of the calsequestrin 2 gene (CASQ2) are at risk for catecholaminergic polymorphic ventricular tachycardia. This mutation (CASQ2D307H) results in decreased ability of CASQ2 to bind Ca2+ in the sarcoplasmic reticulum (SR). In this theoretical study, we investigate a potential mechanism by which CASQ2D307H manifests its pro-arrhythmic consequences in patients. Methods: Using simulations in a model of the guinea pig ventricular myocyte, we investigate the mutation''s effect on SR Ca2+ storage, the Ca2+ transient (CaT), and its indirect effect on ionic currents and membrane potential. We model the effects of isoproterenol (ISO) on CaV1.2 (the L-type Ca2+ current, ICa(L)) and other targets of β-adrenergic stimulation. Results: ISO increases ICa(L), prolonging action potential (AP) duration (Control: 172 ms, +ISO: 207 ms, at cycle length of 1500 ms) and increasing CaT (Control: 0.79 μM, +ISO: 1.61 μM). ISO increases ICa(L) by reducing the fraction of channels which undergo voltage-dependent inactivation and increasing transitions from a non-conducting to conducting mode of channel gating. CASQ2D307H reduces SR storage capacity, thereby reducing the magnitude of CaT (Control: 0.79 μM, CASQ2D307H: 0.52 μM, at cycle length of 1500 ms). The combined effect of CASQ2D307H and ISO elevates SR free Ca2+ at a rapid rate, leading to store-overload-induced Ca2+ release and delayed afterdepolarization (DAD). If resting membrane potential is sufficiently elevated, the Na+–Ca2+ exchange-driven DAD can trigger INa and ICa(L) activation, generating a triggered arrhythmogenic AP. Conclusions: The CASQ2D307H mutation manifests its pro-arrhythmic consequences due to store-overload-induced Ca2+ release and DAD formation due to excess free SR Ca2+ following rapid pacing and β-adrenergic stimulation. [Copyright &y& Elsevier]

Abstract: The multifunctional Ca2+ calmodulin-dependent protein kinase II (CaMKII) has emerged as a pro-arrhythmic signaling molecule. CaMKII can participate in arrhythmia signaling by effects on ion channel proteins, intracellular Ca2+ uptake and release, regulation of cell death, and by activation of hypertrophic signaling pathways. The pleuripotent nature of CaMKII is reminiscent of another serine–threonine kinase, protein kinase A (PKA), which shares many of the same protein targets and is the downstream kinase most associated with β-adrenergic receptor stimulation. The ability of CaMKII to localize and coordinate activity of multiple protein targets linked to Ca2+ signaling set CaMKII apart from other “traditional” arrhythmia drug targets, such as ion channel proteins. This review will discuss some of the biology of CaMKII and focus on work that has been done on molecular, cellular, and whole animal models that together build a case for CaMKII as a pro-arrhythmic signal and as a potential therapeutic target for arrhythmias and structural heart disease. [Copyright &y& Elsevier]

Abstract: Increased consumption of fish rich in omega-3 polyunsaturated fatty acids (ω3-PUFAs) is associated with decreased incidence of sudden cardiac death in post-myocardial infarction patients, but is also related to increased incidence of sudden death and arrhythmias in patients with acute myocardial ischemia. This review discusses the possible pro- and antiarrhythmic mechanisms of ω3-PUFAs in relation to various cardiac pathologies. Differences between circulating and incorporated ω3-PUFAs with respect to electrophysiology are emphasized. We conclude that ω3-PUFAs alter cardiac electrophysiology and thereby may be pro- or antiarrhythmic, dependent on the mechanism of arrhythmia. As ω3-PUFAs may be antiarrhythmic under conditions that favour triggered activity, they may facilitate re-entrant arrhythmias. This may explain the contradictory outcomes of increased intake of fish oil on sudden death and arrhythmias in clinical trials. Advice to increase intake of ω3-PUFA supplements or fatty fish should be tailored to individual patients with respect to the arrhythmogenic mechanisms associated with the underlying pathology. [Copyright &y& Elsevier]

Abstract: Background: Brugada syndrome (BrS) is an inherited electrical disorder associated with a high incidence of sudden death. In a minority of patients, it has been linked to mutations in SCN5A, the gene encoding the pore-forming α-subunit of the cardiac Na+ channel. Other causally related genes still await identification. We evaluated the role of HERG (KCNH2), which encodes the α-subunit of the rapid delayed rectifier K+ channel (I Kr), in BrS. Methods and results: In two unrelated SCN5A mutation-negative patients, different amino acid changes in the C-terminal domain of the HERG channel (G873S and N985S) were identified. Voltage-clamp experiments on transfected HEK-293 cells show that these changes increase I Kr density and cause a negative shift of voltage-dependent inactivation, resulting in increased rectification. Action potential (AP) clamp experiments reveal increased transient HERG peak currents (I peak) during phase-0 and phase-1 of the ventricular AP, particularly at short cycle length. Computer simulations demonstrate that the increased I peak enhances the susceptibility to loss of the AP-dome typically in right ventricular subepicardial myocytes, thereby contributing to the BrS phenotype. Conclusion: Our study reveals a modulatory role of I Kr in BrS. These findings may provide better understanding of BrS and have implications for diagnosis and therapy. [Copyright &y& Elsevier]

Abstract: Objective: We describe a genetic basis for atrial fibrillation and short QT syndrome in utero. Heterologous expression of the mutant channel was used to define the physiological consequences of the mutation. Methods: A baby girl was born at 38 weeks after induction of delivery that was prompted by bradycardia and irregular rythm. ECG revealed atrial fibrillation with slow ventricular response and short QT interval. Genetic analysis identified a de novo missense mutation in the potassium channel KCNQ1 (V141M). To characterize the physiological consequences of the V141M mutation, Xenopus laevis oocytes were injected with cRNA encoding wild-type (wt) KCNQ1 or mutant V141M KCNQ1 subunits, with or without KCNE1. Results: Ionic currents were recorded using standard two-microelectrode voltage clamp techniques. In the absence of KCNE1, wtKCNQ1 and V141M KCNQ1 currents had similar biophysical properties. Coexpression of wtKCNQ1 + KCNE1 subunits induced the typical slowly activating and voltage-dependent delayed rectifier K+ current, I Ks. In contrast, oocytes injected with cRNA encoding V141M KCNQ1 + KCNE1 subunits exhibited an instantaneous and voltage-independent K+-selective current. Coexpression of V141M and wtKCNQ1 with KCNE1 induced a current with intermediate biophysical properties. Computer modeling showed that the mutation would shorten action potential duration of human ventricular myocytes and abolish pacemaker activity of the sinoatrial node. Conclusions: The description of a novel, de novo gain of function mutation in KCNQ1, responsible for atrial fibrillation and short QT syndrome in utero indicates that some of these cases may have a genetic basis and confirms a previous hypothesis that gain of function mutations in KCNQ1 channels can shorten the duration of ventricular and atrial action potentials. [Copyright &y& Elsevier]

Abstract: Objective: Congenital long QT syndrome type 3 (LQT3) is an inherited cardiac arrhythmia disorder due to mutations in the cardiac sodium channel gene, SCN5A. Although most LQT3 mutations cause a persistent sodium current, increasing diversity in the disease mechanism is shown. Here we present the electrophysiological properties of the A1330T sodium channel mutation (DIIIS4–S5 linker). Like the A1330P, LQT3 mutation, A1330T, causes LQT3 in the absence of a persistent current. Methods: A1330T, A1330P and wild-type sodium channels were expressed in HEK-293 cells and characterized using the whole-cell configuration of the patch-clamp technique. Results: The A1330T mutation shifts positively the voltage-dependence of inactivation and speeds recovery from inactivation. Measurements of sodium window (I Na, window) currents revealed a positive shift of the I Na, window voltage range for both 1330 mutants, with in addition an increase in I Na, window magnitude for the A1330P mutant. Action potential (AP) clamp experiments revealed that these changes in I Na, window properties cause an increased inward current during the initial part of phase 4 repolarization of the AP. Conclusions: Our findings indicate that the alanine at position 1330 in the DIIIS4–S5 linker of the cardiac sodium channel has a role in channel fast inactivation. Substitution by a threonine shifts the voltage range of I Na, window activity to more positive potentials. Here the counter-acting effect of outward K+ current is reduced and may delay AP repolarization, explaining the LQT3 phenotype. [Copyright &y& Elsevier]

Abstract: Objective: Mutations in the KCNH2 (hERG, human ether-à-go-go related gene) gene may cause a reduction of the delayed rectifier current I Kr, thereby leading to the long QT syndrome (LQTS). The reduced I Kr delays the repolarisation of cardiac cells and renders patients vulnerable to ventricular arrhythmias and sudden death. We identified a novel mutation in a LQTS family and investigated its functional consequences using molecular and microscopic techniques. Methods and results: Genetic screening in the LQTS family revealed a heterozygous frameshift mutation p.Pro872fs located in the C-terminus of the KCNH2 gene. The mutation leads to a premature truncation of the C-terminus of the hERG protein. p.Pro872fs channels lack 282 amino acids at the C-terminus and possess an extra 4-amino acid tail. Both the kinetic and biochemical properties of the p.Pro872fs and p.Pro872fs/WT channels were studied in HEK293 cells and resulted in a novel proof of concept for heterozygous LQTS mutations: homotetrameric p.Pro872fs channels displayed near-normal expression, trafficking, and channel kinetics. Unexpectedly, upon co-expression of p.Pro872fs and WT channels, the repolarising power (the proportion of hERG current contributing to the action potential as the percentage of the total current available) was substantially higher during action potential clamp experiments as compared to WT channels alone. This would lead to a shorter rather than a prolonged QT interval. However, at the same time, heterotetramerisation of p.Pro872fs and WT channels also caused a dominant negative effect on trafficking by an increase in ER retention of these heterotetrameric channels, which surpassed the former gain in repolarising power. Conclusion: The LQTS phenotype in the studied family is caused by a mutation with novel properties. We demonstrate that a KCNH2 mutation that clinically leads to long QT syndrome causes at the cellular level both a “gain” and a “loss” of HERG channel function due to a kinetic increase in repolarising power and a decrease in trafficking efficiency of heteromultimeric channels. [Copyright &y& Elsevier]

Abstract: The last decade has seen a dramatic increase in the understanding of the molecular basis of arrhythmias. Much of this new information has been driven by genetic studies that focused on rare, monogenic arrhythmia syndromes that were accompanied or followed by cellular electrophysiological or biochemical studies. The marked clinical heterogeneity known from these familial arrhythmia syndromes has led to the development of a multifactorial (“multi-hit”) concept of arrhythmogenesis in which causal gene mutations have a major effect on disease expression that is further modified by other factors such as age, gender, sympathetic tone, and environmental triggers. Systematic genetic studies have unraveled an unexpected DNA sequence variance in these arrhythmia genes that has ethnic-specific patterns. Whether this genetic variance may contribute as a second genetic modifier for arrhythmia development is under current investigation. The aim of this article is to review common genetic variation in ion channel genes and to compare these recent findings. [Copyright &y& Elsevier]

Abstract: After its recognition as a distinct clinical entity, Brugada syndrome is increasingly recognized worldwide as an important cause of sudden cardiac death. Brugada syndrome exhibits autosomal dominant inheritance with SCN5A, which encodes the cardiac sodium channel, as the only gene with a proven involvement in 20–30% of patients. Its signature feature is ST segment elevation in right precordial ECG leads and predisposition to malignant ventricular tachyarrhythmias. The pathophysiological mechanism of ST elevation and ventricular tachyarrhythmia, two phenomena strongly related, is controversial. Here, we review clinical and experimental studies as they provide evidence to support or disprove the two hypotheses on the mechanism of Brugada syndrome that currently receive the widest support: (1) nonuniform abbreviation of right ventricular epicardial action potentials (“repolarization disorder”), (2) conduction delay in the right ventricular outflow tract (“depolarization disorder”). We also propose a schematic representation of the depolarization disorder hypothesis. Moreover, we review recent evidence to suggest that other derangements may also contribute to the pathophysiology of Brugada syndrome, in particular, right ventricular structural derangements. In reviewing these studies, we conclude that, similar to most diseases, it is likely that Brugada syndrome is not fully explained by one single mechanism. Rather than adhering to the notion that Brugada syndrome is a monofactorial disease, we should aim for clarification of the contribution of various pathophysiological mechanisms in individual Brugada syndrome patients and tailor therapy considering each of these mechanisms. [Copyright &y& Elsevier]

Abstract: Objective: In the type 3 long QT syndrome (LQT3), arrhythmia events tend to occur at rest or during sleep. One of the mutations, R1623Q, is located in the voltage sensor of the cardiac sodium channel (hH1), and patients with R1623Q mutation have been also reported to show bradycardia-dependent cardiac events. Although the mutant channel has been characterized by inactivation gating defects, the intrinsic mechanism(s) that might explain why arrhythmia attack is most prevalent at slower heart rates has not been investigated. Methods: cDNA encoding either the wild-type or the R1623Q mutant of hH1 was stably transfected into HEK293 cells. INa was recorded using a whole-cell patch-clamp technique at 23 °C. Results: A train of 50 depolarizing pulses from holding potentials (−120 and −80 mV) to −20 mV or a train of 50 action potential waveforms was applied at different frequencies. When using a rectangular waveform voltage clamp protocol, rate-dependent reduction of INa was holding voltage-dependent but was not different between peak INa and late INa. However, using the action potential clamp, preferential rate-dependent reduction of the phase 3 INa was obvious as compared with peak INa. The discrepancy in the rate-dependent reduction between protocols was attributed to accelerated recovery from inactivation under non-equilibrium condition. Conclusion: The rate dependency of phase 3 INa under non-equilibrium gating is a novel mechanism to explain the enhanced rate-dependent QT-shortening in LQT3 patients. Our findings are important for genotype–phenotype correlations in LQT3 mutants as well as for understanding the function of S4 segment of domain IV region in the cardiac Na+ channel. [Copyright &y& Elsevier]

Background: The human cardiac SCN5A gene encodes for the α subunit of the human cardiac voltage-dependent sodium channel hNav1.5 [Neuron 28 (2) (2000) 365] and carries inward Na current (INa). Mutations in SCN5A cause arrhythmia syndromes including Brugada syndrome (BrS) and congenital long QT syndrome subtype 3 (LQT3). Here, we report a trafficking defective BrS-causing SCN5A mutation that was drug-rescued.Methods and Results: A 14-year-old Caucasian male was diagnosed with BrS with typical ECG pattern for BrS and ventricular fibrillation was easily induced. He also had significant HV interval delay (∼65 ms) and high (31 J) defibrillation thresholds (DFTs). Genomic analysis revealed the SCN5A mutation (G1743R). We engineered G1743R into the cardiac Na channel and transfected HEK-293 cells for functional studies. The mutant channel yielded nearly undetectable sodium channel currents. Coexpression with the β1 subunit, or incubation at low temperature did not increase current density. However, mexiletine, a sodium channel blocker, increased current density 93-fold in G1743R, but only twofold in WT.Conclusions: This study identifies an expression-defective BrS mutation in SCN5A with pharmacological rescue. The profoundly decreased sodium current associated with the G1743R suggests a molecular basis for the delayed His-Purkinje conduction and elevated DFTs observed in the proband. Whether the mutant channel may be rescued in vivo by mexiletine and normalize the patient''s electrophysiologic parameters remains to be tested. [Copyright &y& Elsevier]

Objectives: This study evaluated the in vivo electrophysiological effects of a highly selective slow delayed-rectifier K+-current blocker, HMR 1556, to gain insights into the consequences of selectively inhibiting the slow delayed-rectifier current in vivo. Methods: Atrial and ventricular effective refractory periods, sinus node recovery time, Wenckebach cycle-length, atrial fibrillation duration and electrocardiographic intervals were measured before and after intravenous HMR 1556. Results: HMR 1556 increased atrial and ventricular refractory periods (e.g. by 6±4% and 27±6% at cycle lengths of 360 and 400 ms, respectively), QT intervals and sinus-node recovery times. Beta-adrenoceptor blockade with nadolol abolished all effects except those on ventricular refractoriness and changed positive use-dependent effects on refractoriness to reverse use-dependent ones. In the presence of dofetilide to block rapid delayed-rectifier current, HMR 1556 effects were potentiated (e.g. atrial and ventricular refractory periods increased by 26±3% and 34±3% at cycle lengths of 360 and 400 ms, respectively). HMR 1556 reduced vagal atrial fibrillation duration from 1077±81 to 471±38 s, an effect abolished by nadolol and greatly potentiated by dofetilide (duration 77±30 s). HMR 1556 increased Wenckebach cycle length only in the presence of dofetilide. Conclusions: Slowed delayed-rectifier current inhibition affects atrial repolarization, sinus node function and atrial fibrillation in vivo, but only in the presence of intact beta-adrenergic tone, and delays ventricular repolarization even when beta-adrenoceptors are blocked. The slow delayed-rectifier current is particularly important when rapid delayed-rectifier current is suppressed, illustrating the importance of repolarization reserve. [Copyright &y& Elsevier]

Objective: In German shepherd dogs having inherited arrhythmias and sudden death, pause-dependent arrhythmias are triggered by early afterdepolarizations (EADs) originating from left ventricular (LV) Purkinje fibers (PF). Heterogeneity of LV repolarization provides the arrhythmogenic substrate. To elucidate the mechanisms whereby α-adrenergic stimulation exacerbates these arrhythmias we tested the effects of phenylephrine on both arrhythmogenic trigger and substrate. Methods and results: We used microelectrode techniques to record action potentials from LV and right ventricular (RV) PF and from midmyocardial sections of anteroseptal, anterobasal and posterobasal LV wall of unafflicted and afflicted dogs. EADs occurred spontaneously in 8 of 12 LV PF and in no RV PF from afflicted dogs and in no PF from unafflicted dogs. In LV PF from afflicted dogs, phenylephrine (10−9–10−5 M) concentration-dependently decreased membrane potential, induced abnormal automaticity at membrane potentials from −65 to −45 mV in 6 LV PF and potentiated EADs in another 6. To determine the mechanisms of membrane depolarization we studied phenylephrine effects on IK1 in voltage-clamped single LV and RV PF cells from afflicted dogs. In LV PF, phenylephrine (10−5 M) reduced IK1 over the range of −120 to −40 mV and had no effects on RV PF. Regional heterogeneity of LV repolarization was observed in afflicted dogs only. Phenylephrine had no effects on repolarization in either group. Conclusion(s): α-adrenergic stimulation exacerbates arrhythmias in afflicted dogs by increasing the arrhythmogenic trigger while leaving the substrate unchanged. Decrease in IK1 contributes importantly to α-adrenergic effects on LV PF. [Copyright &y& Elsevier]

This review focuses mainly on studies in non-ischemic animal models of heart failure. These animals develop ventricular arrhythmias, mostly non-sustained ventricular tachycardia, and often die suddenly. Clinical studies suggest that sudden death is due to ventricular tachycardia or fibrillation in about 50% of cases, the other half to bradyarrhythmias or electromechanical dissociation.Electrophysiologic changes in heart failure are not confined to the ventricles: the intrinsic sinus rate is reduced due to a downregulation of If and sensitivity to acetylcholine is enhanced by upregulation of the muscarinic receptor. Reduction of heart rate may be a protective mechanism since at rapid rates contractility is reduced and the likelihood for triggered activity due to delayed afterdepolarizations is enhanced. The beneficial effect of β-adrenergic blockade in patients may be partly due to the reduction in sinus rate.Although the results of different studies often vary, the most consistent electrophysiological changes in the ventricles are prolongation of the action potential, especially at slow rates, a reduction in the transient outward current Ito, the rapid and slow components of the delayed rectifier Ikr and Iks, and the inward rectifier Ik1. Abnormalities in intracellular calcium handling play a major role in the genesis of delayed afterdepolarizations. Triggered activity based on delayed afterdepolarizations has been demonstrated in failing myocardium and are caused by spontaneous release of calcium from the sarcoplasmic reticulum (SR), especially in the presence of noradrenaline. Three factors combine to the enhanced propensity for the occurrence of delayed afterdepolarizations: (1) increased activity of the Na/Ca exchanger, (2) a reduced inward rectifier, (3) residual β-adrenergic responsiveness required to raise the reduced sarcoplasmic calcium content to a level where spontaneous calcium release occurs.Early afterdepolarizations have also been demonstrated, especially in human myocytes from failing hearts in the presence of noradrenaline.Mapping experiments have shown that the ventricular arrhythmias are mainly due to non-reentrant mechanisms, most likely triggered activity based on delayed afterdepolarizations. [Copyright &y& Elsevier]

Human hereditary long QT syndrome is a cardiac disease characterized by prolongation of the QT interval and increased susceptibility to ventricular arrhythmias and sudden cardiac death. Mutations in the human-ether-a-go-go-related gene (hERG), encoding the protein underlying the repolarizing cardiac IKr potassium current, cause chromosome 7-linked long QT syndrome 2. Loss of function of mutant hERG channels may be caused by several mechanisms, including altered current kinetics, altered ion selectivity, or defective intracellular protein trafficking. Especially the latter category has become a focus of particular interest recently, because some of the mutant subunits display wild type current properties when normal trafficking is restored and channels are inserted in the cell membrane in vitro. This review summarizes the current knowledge on hERG channel trafficking under physiological and pathological conditions. In addition, therapeutic approaches to restore normal hERG trafficking in vitro and in vivo are discussed. [Copyright &y& Elsevier]

Objective: Chronic beta-adrenoceptor antagonist (β-blocker) treatment reduces the incidence of reversion to AF in patients, possibly via an adaptive myocardial response. However, the underlying electrophysiological mechanisms are presently unclear. We aimed to investigate electrophysiological changes in human atrial cells associated with chronic treatment with β-blockers and other cardiovascular-acting drugs. Methods: Myocytes were isolated enzymatically from the right atrial appendage of 40 consenting patients who were in sinus rhythm. The cellular action potential duration (APD), effective refractory period (ERP), L-type Ca2+ current (ICaL), transient (ITO) and sustained (IKSUS) outward K+ currents, and input resistance (Ri) were recorded using the whole cell patch clamp. Drug treatments and clinical characteristics were compared with electrophysiological measurements using simple and multiple regression analyses. P<0.05 was taken as statistically significant. Results: In atrial cells from patients treated chronically with β-blockers, the APD90 and ERP (75 beats/min stimulation) were significantly longer, at 213±11 and 233±11 ms, respectively (n=15 patients), than in cells from non-β-blocked patients, at 176±12 and 184±12 ms (n=11). These cells also displayed a significantly reduced action potential phase 1 velocity (22±3 vs. 34±3 V/s). Chronic β-blockade was also associated with a significant reduction in the heart rate (58±3 vs. 69±5 beats/min) and in the density of ITO (8.7±1.3 vs. 13.7±2.1 pA/pF), an increase in the Ri (214±24 vs. 132±14 MΩ), but no significant change in ICaL or IKSUS. The ITO blocker 4-aminopyridine largely mimicked the changes in phase 1 and ERP associated with chronic β-blockade, in cells from non-β-blocked patients. Chronic treatment of patients with calcium channel blockers or angiotensin converting enzyme inhibitors (n=11–13 patients) was not associated with any significant changes in atrial cell electrophysiology. Conclusion: The observed atrial cellular electrophysiological changes associated with chronic β-blockade are consistent with a long-term adaptive response, a type of ‘pharmacological remodelling’, and provide mechanistic evidence supportive of the anti-arrhythmic actions of β-blockade. [Copyright &y& Elsevier]

Objective: To attempt to determine the relative value of preclinical cardiac electrophysiology data (in vitro and in vivo) for predicting risk of torsade de pointes (TdP) in clinical use. Methods: Published data on hERG (or IKr) activity, cardiac action potential duration (at 90% repolarisation; APD90), and QT prolongation in dogs were compared against QT effects and reports of TdP in humans for 100 drugs. These data were set against the free plasma concentrations attained during clinical use (effective therapeutic plasma concentrations; ETPCunbound). The drugs were divided into five categories: (1) Class Ia and III antiarrhythmics; (2) Withdrawn from market due to TdP; (3) Measurable incidence/numerous reports of TdP in humans; (4) Isolated reports of TdP in humans; (5) No reports of TdP in humans. Results: Data from hERG (or IKr) assays in addition to ETPCunbound data were available for 52 drugs. For Category 1 drugs, data for hERG/IKr IC50, APD90, QTc in animals and QTc in humans were generally close to or superimposed on the ETPCunbound values. This relationship was uncoupled in the other categories, with more complex relationships between the data. In Category 1 (except amiodarone), the ratios between hERG/IKr IC50 and ETPCunbound (max) ranged from 0.1- to 31-fold. Similar ranges were obtained for drugs in Category 2 (0.31- to 13-fold) and Category 3 (0.03- to 35-fold). A large spread was found for Category 4 drugs (0.13- to 35 700-fold); this category embraced an assortment of mechanisms ranging from drugs which may well be affecting IKr currents in clinical use (e.g. sparfloxacin) to others such as nifedipine (35 700-fold) where channel block is not involved. Finally, for the majority of Category 5 drugs there was a >30-fold separation between hERG/IKr activity and ETPCunbound values, with the notable exception of verapamil (1.7-fold), which is free from QT prolongation in man; this is probably explained by its multiple interactions with cardiac ion channels. Conclusions: The dataset confirms the widely-held belief that most drugs associated with TdP in humans are also associated with hERG K+ channel block at concentrations close to or superimposed upon the free plasma concentrations found in clinical use. A 30-fold margin between Cmax and hERG IC50 may suffice for drugs currently undergoing clinical evaluation, but for future drug discovery programmes, pharmaceutical companies should consider increasing this margin, particularly for drugs aimed at non-debilitating diseases. However, interactions with multiple cardiac ion channels can either mitigate or exacerbate the prolongation of APD and QT that would ensue from block of IKr currents alone, and delay of repolarisation per se is not necessarily torsadogenic. Clearly, an integrated assessment of in vitro and in vivo data is required in order to predict the torsadogenic risk of a new candidate drug in humans. [Copyright &y& Elsevier]

The serum and glucocorticoid inducible kinase SGK1 and its isoform SGK3 are both expressed in cardiac tissue. One of the functions of SGK1 is the phosphorylation and inactivation of the ubiquitin ligase Nedd4-2, which in turn could be shown to downregulate the voltage-gated Na+ channel SCN5A (hH1). The present study has been performed to test for a role of SGK1 and SGK3 in the regulation of SCN5A. To this end cRNA encoding the human Na+ channel SCN5A was injected into Xenopus laevis oocytes with or without cRNA encoding the wild-type kinases SGK1, the constitutively active kinase S422DSGK1, the inactive form K127NSGK1 or the wild-type SGK3. SCN5A currents were activated by coexpression of either wild-type SGK1 or SGK3 or the constitutively active S422DSGK1. In contrast, the inactive mutant K127NSGK1 significantly decreased the currents. Moreover, coexpression of SGK3 significantly altered SCN5A gating, i.e. it hyperpolarized the activation threshold and depolarized the prepotential required for 50% availability of the channel. Opposite shifts of gating properties were elicited by mutation of serine to alanine (S483ASCN5A and S663ASCN5A) in the SGK consensus sequences of SCN5A. The present observations disclose a role of the kinases SGK1 and SGK3 in the regulation of cardiac Na+ channels. As SGK1 is upregulated by glucocorticoids, mineralocorticoids and a variety of inflammatory mediators and both kinases are activated by insulin and IGF1, the kinases could mediate effects of those hormones and mediators on cardiac function. [Copyright &y& Elsevier]

Objectives: We hypothesized that exposure of N629D/wildtype channels to transient increases in [K+]o could alter the conformation of the outer vestibule and thus reverse the disease phenotype. N629D is a recently described mutation of the HERG1 gene that causes familial long QT syndrome. This mutation alters the pore signature sequence resulting in loss of K+ selectivity. Previous studies have reported that enforced occupancy of [K+]o at sites near the selectivity filter alters the conformation/folding of the outer vestibule of the Kv2.1 channel. Methods: Since the long QT syndrome is manifest in individuals who are heterozygous for this HERG trait, we co-expressed N629D and the wildtype at equimolar concentrations. Results: Co-expression of N629D/wildtype in Xenopus oocytes and mammalian cells resulted in a channel with a positive shift in reversal potential and a loss in the outward tail current, relative to the wildtype. Exposure of the N629D/wildtype to transient increases in [K+]o from 5 to 40 mM/l changed the tail current from inward to outward during repolarization and restored the reversal potential to values similar to the wildtype. These findings in Xenopus oocytes were also seen when N620D/wildtype channels were expressed in mammalian cells. These [K+]o-dependent changes persisted for hours after the [K+]o was returned to 2.5 mM. This potential therapeutic effect began with increases in [K+]o from 2.5 to 5 mM. Conclusions: This study reports a novel therapeutic strategy and mechanism to partially restore physiologic function in this HERG LQTS mutation. [Copyright &y& Elsevier]

Objectives: Propafenone is a class Ic antiarrhythmic drug used to maintain sinus rhythm in patients with atrial fibrillation. During chronic therapy, it undergoes extensive first-pass hepatic metabolism to 5-hydroxypropafenone. In the present study we have analysed the effects of propafenone and 5-hydroxypropafenone on HERG current. Methods: The whole-cell configuration of the patch-clamp technique was used in CHO cells stably transfected with the gene encoding HERG channels. Results: Propafenone and 5-hydroxypropafenone (2 μM) inhibited HERG current by 78.7±2.3% (n=7) and 71.1±4.1% (n=7, P>0.05) when measured at the end of 5-s depolarizing pulses to −10 mV. Block measured at the maximum peak of tail currents recorded at −60 mV was similar for propafenone (78.3±2.0%, n=7, P>0.05) and higher for 5-hydroxypropafenone (79.3±1.5%, n=7, P<0.05). Propafenone and 5-hydroxypropafenone shifted the midpoint of the activation curve by −10.2±0.9 mV (n=7, P<0.01) and −7.4±1.1 mV (n=10, P<0.01), respectively. Both drugs accelerated the deactivation and the inactivation process of HERG current. Propafenone, but not 5-hydroxypropafenone, inhibited to a higher extent HERG current at the end of 5-s depolarizing pulses to 0 mV than after promoting the transition of HERG channels from the inactivated to the opened state. Conclusions: These results indicate that propafenone and its main active metabolite, 5-hydroxypropafenone, block HERG channels to a similar extent by binding predominantly to the open state of the channel. [Copyright &y& Elsevier]

Background: We, and others, have previously reported a strong correlation between increased inter-ventricular dispersion of repolarization and the occurrence of fatal arrhythmia in animal models of CHF. The existence of this and other such distinct electrophysiologic phenotypes in right (RV) vs. left ventricles (LV) could be explained by chamber-specific patterns of gene expression. Methods: We employed microarray gene profiling of 13 824 sequence-verified, nonredundant rodent cDNAs to compare myocardial gene expression in RV vs. LV of rats with surgically induced myocardial infarction (MI: n=3) and in sham-operated animals (Sham: n=3). Results: Significant LV infarction (32±4% LV) and severe CHF were observed in all MI animals at 4 weeks. In Sham animals, 937 genes exhibited significant differential expression in RV vs. LV myocardium. In MI animals, 1158 genes exhibited significant differential expression in RV vs. LV. Of those genes exhibiting significant differential expression, only 241 were common to both Sham and MI animals. Differentially expressed genes included those involved in signal transduction, cell growth and maintenance, and apoptosis. Genes with potential roles in altered dispersion of repolarization included voltage-dependent Ca2+ channel γ subunit (MI 8-fold↑) and K+ inwardly rectifying channel subfamily J, member 10 (MI 6-fold↓). Gap junction membrane channel protein α 4 (MI 6-fold↓) and cardiac troponin I (MI 8-fold↓) were also significantly differentially expressed. Inter-ventricular comparisons revealed significantly greater alterations in gene expression vs. intra-ventricular comparisons. Conclusions: Microarray gene profiling has revealed candidate genes, some of them novel, which may account for chamber-specific ventricular electrophysiologic phenotypes, both in physiologic as well as in arrhythmogenic states such as CHF. [Copyright &y& Elsevier]

Objective: In various cell types, membrane-associated guanylate kinases proteins called MAGUK play a major role in the spatial localization and clustering of ion channels. Here, we studied the expression and role of these anchoring proteins in human right atrial myocardium by means of various molecular, biochemical and physiological methods. Methods and results: SAP-97, PSD-95, Chapsyn and SAP-102 messengers were detected by reverse transcriptase–polymerase chain reaction (RT-PCR) on mRNA extracted from both whole myocardium and isolated myocytes. Western blot revealed that the MAGUK protein SAP-97 and, to a lesser extent, PSD-95, is abundantly expressed in human atrial myocardium, while Chapsyn are almost undetectable. Confocal microscopic visualization of cryosection of atrial myocardium stained with the anti-PSD-95 family antibody showed positive staining at the plasma membrane level and cell extremity. Calpain-I cleaved both SAP-97 and PSD-95 proteins, resulting in an accumulation of short bands, including an 80-kDa band that was also detected in the cytosolic protein fraction. Immunoprecipitation of SAP-97 co-precipitated hKv1.5 channels, and vice versa. Co-expression of cloned SAP-97 and hKv1.5 channels in Chinese hamster ovarian (CHO) cells increased the K+ current (157.00±19.45 pA/pF vs. 344.50±58.58 pA/pF at +50 mV). Conclusions: The protein SAP-97 is abundantly expressed in human atrial myocardium in association with hKv1.5 channels, and probably contributes to regulating the functional expression of the latter. [Copyright &y& Elsevier]

Objective: Co-expression of the KvLQT1 and minK potassium channel subunits is required to recapitulate IKs, the slow component of the cardiac delayed rectifier current, and mutations in either gene cause the congenital Long QT syndrome. It is becoming increasingly well-recognized that multiprotein channel complexes containing proteins capable of modulating channel function assemble at the plasma membrane. Thus, the aim of our study was to identify proteins involved in IKs modulation. Methods and results: Using a yeast-two-hybrid screen with the intracytoplasmic C-terminus of minK as bait, we identified the cardiac-enriched four-and-a-half LIM domain-containing protein (fhl2) as a potential minK partner. We show interaction between the two proteins in GST pulldown assays and demonstrate overlapping subcellular localization using immunocytochemistry of transfected cells supporting a potential interaction. At the functional level, expression of KvLQT1and minK in HEK cells, which lack endogenous fhl2 protein, generated IKs only when fhl2 was co-expressed. By contrast, in CHO-K1 cells, which express fhl2 endogenously, IKs was suppressed by anti-fhl2 antisense which did not affect the currents generated by KvLQT1alone. Conclusion: These data indicate that at least in heterologous cells, the generation of IKs requires fhl2 as an additional protein component. [Copyright &y& Elsevier]