Probing the molecular and structural basis of voltage sensor gating in Kv11.1 ion channels

Congenital mutations in the cardiac Kv11.1 channel can cause long QT syndrome type 2 (LQTS2), a heart rhythm disorder associated with sudden cardiac death. Mutations act either by reducing protein expression at the membrane, and/or by perturbing the intricate gating properties of Kv11.1 channels. A number of clinical LQTS2-associated mutations have been reported in the first transmembrane segment (S1) of Kv11.1 channels but the role of this region of the channel is largely unexplored. In part this is due to problems defining the extent of the S1 helix, as a consequence of its low sequence homology with other Kv family members. Here we used NMR spectroscopy and electrophysiological characterization to show that the S1 of Kv11.1 channels extends seven helical turns, from Pro405 to Phe431, and is flanked by unstructured loops. Functional analysis suggests that pre-S1 loop residues His402 and Tyr403 play an important role in regulating the kinetics and voltage dependence of channel activation and deactivation. Multiple residues within the S1 helix also play an important role in fine-tuning the voltage dependence of activation, regulating slow deactivation, and modulating C-type inactivation of Kv11.1 channels. We demonstrate that for Kv11.1, activation and deactivation processes are not simple reversal transitions, but rather deactivation and inactivation processes are likely to be coupled. Analyses of LQTS2-associated mutations in the pre-S1 loop or S1 helix of Kv11.1 channels demonstrate perturbations to both protein expression and most gating transitions. Thus S1 region mutations would reduce both the action potential repolarizing current passed by Kv11.1 channels in cardiac myocytes, as well as the current passed in response to premature depolarizations that normally helps protect against the formation of ectopic beats

Murine Electrophysiological Models of Cardiac Arrhythmogenesis

Cardiac arrhythmias can follow disruption of the normal cellular electrophysiological processes underlying excitable activity and their tissue propagation as coherent wavefronts from the primary sinoatrial node pacemaker, through the atria, conducting structures and ventricular myocardium. These physiological events are driven by interacting, voltage-dependent, processes of activation, inactivation, and recovery in the ion channels present in cardiomyocyte membranes. Generation and conduction of these events are further modulated by intracellular Ca$^{2+}$ homeostasis, and metabolic and structural change. This review describes experimental studies on murine models for known clinical arrhythmic conditions in which these mechanisms were modified by genetic, physiological, or pharmacological manipulation. These exemplars yielded molecular, physiological, and structural phenotypes often directly translatable to their corresponding clinical conditions, which could be investigated at the molecular, cellular, tissue, organ, and whole animal levels. Arrhythmogenesis could be explored during normal pacing activity, regular stimulation, following imposed extra-stimuli, or during progressively incremented steady pacing frequencies. Arrhythmic substrate was identified with temporal and spatial functional heterogeneities predisposing to reentrant excitation phenomena. These could arise from abnormalities in cardiac pacing function, tissue electrical connectivity, and cellular excitation and recovery. Triggering events during or following recovery from action potential excitation could thereby lead to sustained arrhythmia. These surface membrane processes were modified by alterations in cellular Ca$^{2+}$ homeostasis and energetics, as well as cellular and tissue structural change. Study of murine systems thus offers major insights into both our understanding of normal cardiac activity and its propagation, and their relationship to mechanisms generating clinical arrhythmias.Medical Research Council, Wellcome Trust, McVeigh Benefaction, British Heart Foundation, Helen Kirkland Trust, Sudden Arrhythmic Death Syndrome - SADS U

Biophysical Characterization of Three SCN5A Mutations Linked to Long QT Syndrome Type 3, Sudden Infant Death Syndrome, and Atrial Fibrillation

Le gène SCN5A encode la sous-unité principale du canal sodique cardiaque (Nav1.5). Ce canal est responsable de l'initiation et de la propagation du potentiel d'action cardiaque. Un dysfonctionnement de ce canal peut causer le syndrome du QT long de type 3 (LQT3) et la fibrillation auriculaire (AF). Les patients atteints du LQT3 sont à risques de développer des arythmies létales, particulièrement des torsades de pointes qui peuvent causer le syndrome de mort subite du nourrisson (SIDS). Objectifs : Le but de cette étude est de caractériser les propriétés biophysiques de trois mutations sur le gène SCN5A : Y1767C, S1333Y et K1493R. Ces trois mutations ont respectivement été retrouvées chez un patient souffrant du LQT3, chez un patient mort du SIDS et la dernière mutation chez un patient souffrant d'AF. Méthodes : Des cellules tsA 201 ont été transfectées avec le gène codant pour le canal sauvage et les gènes codant pour les canaux mutés. Par la suite, leurs caractéristiques biophysiques ont été étudiées par la méthode du patch-clamp en configuration cellule entière. Résultats : La mutation Y1767C est située dans le segment 6 du domaine IV (DIVS6). Cette mutation sur le canal produit un courant persistant et un courant de fenêtre augmenté, ces résultats expliquent les phénotypes cliniques des patients affectés de cette mutation. La ranolazine, un nouveau bloqueur des canaux Na+, peut bloquer efficacement le courant Na+ persistant et réduire le courant de fenêtre. Ces canaux mutés montrent aussi une augmentation de l'inhibition fréquence-dépendante ainsi qu'une réactivation lente. La mutation S1333Y est situé sur la boucle S4 et S5 du domaine III. L'étude fonctionnelle de ce canal montre un gain de fonction : un courant Na+ persistant et une augmentation du courant de fenêtre provoquée par un déplacement de -8 mV de l'activation et de +7mV de l'inactivation. La mutation K1493R est située sur la boucle entre les domaines III-IV. Cette mutation provoque un déplacement vers des potentiels plus dépolarisés de l'inactivation est entraîne une augmentation du courant de fenêtre. Conclusion : Les manifestations cliniques observées chez les patients sont probablement dues aux changements des propriétés biophysiques provoqués par les trois mutations sur Nav1.5 rapportées dans cette étude. Nous concluons donc que (1) Y1767C est une mutation provoquant le LQT3. L'effet observé par la ranolazine sur cette mutation (la ranolazine agit probablement comme un bloqueur des canaux ouverts) nous donne de nouveaux indices pour le traitement des patients porteurs de cette mutation. (2) La mort subite du nourrisson observé est probablement lié à un syndrome LQT3 associé à la mutation S1333Y. (3) La mutation K1493R provoque de la fibrillation auriculaire causée par une hyperexcitabilité des cardiomyocytes

Cardiac Arrhythmias

The most intimate mechanisms of cardiac arrhythmias are still quite unknown to scientists. Genetic studies on ionic alterations, the electrocardiographic features of cardiac rhythm and an arsenal of diagnostic tests have done more in the last five years than in all the history of cardiology. Similarly, therapy to prevent or cure such diseases is growing rapidly day by day. In this book the reader will be able to see with brighter light some of these intimate mechanisms of production, as well as cutting-edge therapies to date. Genetic studies, electrophysiological and electrocardiographyc features, ion channel alterations, heart diseases still unknown , and even the relationship between the psychic sphere and the heart have been exposed in this book. It deserves to be read