22 research outputs found

    Cellular Mechanisms of Sinus Node Dysfunction in Carriers of the SCN5A-E161K Mutation and Role of the H558R Polymorphism

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    Background: Carriers of the E161K mutation in the SCN5A gene, encoding the NaV1.5 pore-forming α-subunit of the ion channel carrying the fast sodium current (INa), show sinus bradycardia and occasional exit block. Voltage clamp experiments in mammalian expression systems revealed a mutation-induced 2.5- to 4-fold reduction in INa peak current density as well as a +19 mV shift and reduced steepness of the steady-state activation curve. The highly common H558R polymorphism in NaV1.5 limits this shift to +13 mV, but also introduces a -10 mV shift in steady-state inactivation.Aim: We assessed the cellular mechanism by which the E161K mutation causes sinus node dysfunction in heterozygous mutation carriers as well as the potential role of the H558R polymorphism.Methods: We incorporated the mutation-induced changes in INa into the Fabbri-Severi model of a single human sinoatrial node cell and the Maleckar et al. human atrial cell model, and carried out simulations under control conditions and over a wide range of acetylcholine levels.Results: In absence of the H558R polymorphism, the E161K mutation increased the basic cycle length of the sinoatrial node cell from 813 to 866 ms. In the simulated presence of 10 and 25 nM acetylcholine, basic cycle length increased from 1027 to 1131 and from 1448 to 1795 ms, respectively. The increase in cycle length was the result of a significant slowing of diastolic depolarization. The mutation-induced reduction in INa window current had reduced the contribution of the mutant component of INa to the net membrane current during diastolic depolarization to effectively zero. Highly similar results were obtained in presence of the H558R polymorphism. Atrial excitability was reduced, both in absence and presence of the H558R polymorphism, as reflected by an increase in threshold stimulus current and a concomitant decrease in capacitive current of the atrial cell.Conclusion: We conclude that the experimentally identified mutation-induced changes in INa can explain the clinically observed sinus bradycardia and potentially the occasional exit block. Furthermore, we conclude that the common H558R polymorphism does not significantly alter the effects of the E161K mutation and can thus not explain the reduced penetrance of the E161K mutation

    Cardiac sodium channel, its mutations and their spectrum of arrhythmia phenotypes

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    The mechanisms of cellular excitability and propagation of electrical signals in the cardiac muscle are very important functionally and pathologically. The heart is constituted by three types of muscle: atrial, ventricular, and specialized excitatory and conducting fi bers. From a physiological and pathophysiological point of view, the conformational states of the sodium channel during heart function constitute a signifi cant aspect for the diagnosis and treatment of heart diseases. Functional states of the sodium channel (closed, open, and inactivated) and their structure help to understand the cardiac regulation processes. There are areas in the cardiac muscle with anatomical and functional differentiation that present automatism, thus subjecting the rest of the fi bers to their own rhythm. The rate of these (pacemaker) areas could be altered by modifi cations in ions, temperature and especially, the autonomic system. Excitability is a property of the myocardium to react when stimulated. Another electrical property is conductivity, which is characterized by a conduction and activation process, where the action potential, by the all-or-nothing law, travels throughout the heart. Heart relaxation also stands out as an active process, dependent on the energetic output and on specificion and enzymatic actions, with the role of sodium channel being outstanding in the functional process. In the gene mutation aspects that encode the rapid sodium channel (SCN5A gene), this channel is responsible for several phenotypes, such as Brugada syndrome, idiopathic ventricular fibrillation, dilated cardiomyopathy, early repolarization syndrome, familial atrial fibrillation, variant 3 of long QT syndrome, multifocal ectopic ventricular contractions originating in Purkinje arborizations, progressive cardiac conduction defect (Lenègre disease), sudden infant death syndrome, sick sinus syndrome, sudden unexplained nocturnal death syndrome, among other sodium channel alterations with clinical overlapping. Finally, it seems appropriate to consider the “sodium channel syndrome” (mutations in the gene of the α subunit of the sodium channel, SCN5A gene) as a single clinical entity that may manifest in a wide range of phenotypes, to thus have a better insight on these cardiac syndromes and potential outcomes for their clinical treatment

    Computational modeling of human sinoatrial node: what simulations tell us about pacemaking

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    The Sinoatrial node (SAN) is the primary pacemaker in physiological conditions. SAN tissue is characterized by auto-ryhthmicity, i.e. it does not need external stimuli to initiate its electrical activity. The auto-rhythmic behavior is due to the spontaneous slow depolarization during the diastolic phase. Understanding the biophysical mechanisms at the base of diastolic depolarization is crucial to modulate the heart rate (HR). In turn, HR modulation is fundamental to treat cardiac arrhythmias, so that atria and ventricles can fill and pump the blood properly. The overall aim of the thesis is the investigation of the underlying mechanisms responsible for the pacemaking in human. To this end, a human computational model of the action potential (AP) of the SAN was developed. Pacemaking modulation at single cell level, effects of ion channel mutations on the beating rate and propagation of the electrical trigger from SAN to atrial tissue are the faced topics The human single cell SAN model was developed starting from the rabbit SAN by Severi et al.; the parent model was updated with experimental data and automatic optimization to match the AP features reported in literature. A sensitivity analysis was performed to identify the most influencing parameters. The investigation of pacemaking modulation was carried out through the simulation of current blockade and mimicking the stimulation of the autonomic nervous system. The model was validated comparing the simulated electrophysiological effects due to ion channel mutations on beating rate with clinical data of symptomatic subjects carriers of the mutation. More insights on pacemaking mechanisms were obtained thanks to the inclusion of calcium-activated potassium currents, which link changes in the intracellular calcium to the membrane. Finally, the propagation of the AP from the SAN to the atrial tissue and the source-sink interplay was investigated employing a mono-dimensional strand composed by SAN and atrial models

    Cardiac sodium channelopathies

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    Cardiac sodium channel are protein complexes that are expressed in the sarcolemma of cardiomyocytes to carry a large inward depolarizing current (INa) during phase 0 of the cardiac action potential. The importance of INa for normal cardiac electrical activity is reflected by the high incidence of arrhythmias in cardiac sodium channelopathies, i.e., arrhythmogenic diseases in patients with mutations in SCN5A, the gene responsible for the pore-forming ion-conducting α-subunit, or in genes that encode the ancillary β-subunits or regulatory proteins of the cardiac sodium channel. While clinical and genetic studies have laid the foundation for our understanding of cardiac sodium channelopathies by establishing links between arrhythmogenic diseases and mutations in genes that encode various subunits of the cardiac sodium channel, biophysical studies (particularly in heterologous expression systems and transgenic mouse models) have provided insights into the mechanisms by which INa dysfunction causes disease in such channelopathies. It is now recognized that mutations that increase INa delay cardiac repolarization, prolong action potential duration, and cause long QT syndrome, while mutations that reduce INa decrease cardiac excitability, reduce electrical conduction velocity, and induce Brugada syndrome, progressive cardiac conduction disease, sick sinus syndrome, or combinations thereof. Recently, mutation-induced INa dysfunction was also linked to dilated cardiomyopathy, atrial fibrillation, and sudden infant death syndrome. This review describes the structure and function of the cardiac sodium channel and its various subunits, summarizes major cardiac sodium channelopathies and the current knowledge concerning their genetic background and underlying molecular mechanisms, and discusses recent advances in the discovery of mutation-specific therapies in the management of these channelopathies

    Gain-of-Function Mutations in SCN5A Gene Lead to Type-3 Long QT Syndrome

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    Type-3 long QT syndrome, which is related to type 5 voltage-gated sodium channel alpha subunit (SCN5A) mutation, has been identified since 1995. LQTS mutation in SCN5A is a gain-of-function mutation producing late sodium current, INa,L. Brugada mutation in SCN5A is a loss-of-function causing INa decrease. Whereas, the mechanism for Dilated Cardiomyopathy mutations in SCN5A is still not fully understood. N1325S is one of the first series of mutations identified for type-3 LQTS. Our lab created a mouse model for LQTS by expressing SCN5A mutation N1325S in the mouse hearts (TG-NS) and a matched experimental control line with overexpression of wild- type SCN5A (TG-WT). There are some interesting findings in TG-NS mice: (i) Intracellular sodium (Na+) level is higher in TG-NS myocytes compared with TG-WT myocytes. (ii) Ca2+ handling is abnormal in TG-NS myocytes, but not in TG-WT myocytes. (iii) Apoptosis was also found in TG-NS mouse heart tissue, but not in TG-WT hearts. These results provoke the hypothesis that gain-of-function mutation N1325S in SCN5A leads to LQTS through abnormal cytosolic Ca2+ homeostasis. Another LQTS mutation in SCN5A R1193Q was identified in 2004 and the electrophysiological property is similar to other gain-of-function SCN5A mutations. The transgenic mouse model for this mutation was also established and the surface Electrocardiogram (ECG) results indicate longer corrected QT interval also present in transgenic mice carrying R1193Q mutation. Besides, quinidine, an anti-arrhythmic medication, can cause arrhythmic symptoms such as premature ventricular contraction (PVC), premature atrial contraction (PAC) and atrioventricular (AV) block in R1193Q transgenic mice.In order to further study the relationship between abnormal Ca2+ handling and the type of SCN5A mutation, either gain-of-function or loss-of-function, we have chosen HL-1 cells, a cell line with indefinite passages in culture with all the adult cardiac phenotypes. The similar abnormal Ca2+ handling was also identified in HL-1 cells e

    Sodium Channelopathy Underlying Familial Sick Sinus Syndrome With Early Onset and Predominantly Male Characteristics

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    Background-Sick sinus syndrome (SSS) is a common arrhythmia often associated with aging or organic heart diseases but may also occur in a familial form with a variable mode of inheritance. Despite the identifcation of causative genes, including cardiac Na channel (SCN5A), the pathogenesis and molecular epidemiology of familial SSS remain undetermined primarily because of its rarity. Methods and Results-We genetically screened 48 members of 15 SSS families for mutations in several candidate genes and determined the functional properties of mutant Na channels using whole-cell patch clamping. We identifed 6 SCN5A mutations including a compound heterozygous mutation. Heterologously expressed mutant Na channels showed loss-of-function properties of reduced or no Na current density in conjunction with gating modulations. Among 19 family members with SCN5A mutations, QT prolongation and Brugada syndrome were associated in 4 and 2 individuals, respectively. Age of onset in probands carrying SCN5A mutations was signifcantly less (mean±SE, 12.4±4.6 years; n=5) than in SCN5A-negative probands (47.0±4.6 years; n=10; P<0.001) or nonfamilial SSS (74.3±0.4 years; n=538; P<0.001). Meta-analysis of SSS probands carrying SCN5A mutations (n=29) indicated profound male predominance (79.3%) resembling Brugada syndrome but with a considerably earlier age of onset (20.9±3.4 years). Conclusions-The notable pathophysiological overlap between familial SSS and Na channelopathy indicates that familial SSS with SCN5A mutations may represent a subset of cardiac Na channelopathy with strong male predominance and early clinical manifestations

    Cardiac sodium channelopathies

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