Bruno Gavillet

Cardiac Sodium Channel Nav1.5 Is Regulated by a Multiprotein Complex Composed of Syntrophins and Dystrophin

The cardiac sodium channel Nav1.5 plays a key role in cardiac excitability and conduction. The purpose of this study was to elucidate the role of the PDZ domain-binding motif formed by the last three residues (Ser-Ile-Val) of the Nav1.5 C-terminus. Pull-down experiments were performed using Nav1.5 C-terminus fusion proteins and human or mouse heart protein extracts, combined with mass spectrometry analysis. These experiments revealed that the C-terminus associates with dystrophin, and that this interaction was mediated by alpha- and beta-syntrophin proteins. Truncation of the PDZ domain-binding motif abolished the interaction. We used dystrophin-deficient mdx5cv mice to study the role of this protein complex in Nav1.5 function. Western blot experiments revealed a 50% decrease in the Nav1.5 protein levels in mdx5cv hearts, whereas Nav1.5 mRNA levels were unchanged. Patch-clamp experiments showed a 29% decrease of sodium current in isolated mdx5cv cardiomyocytes. Finally, ECG measurements of the mdx5cv mice exhibited a 19% reduction in the P wave amplitude, and an 18% increase of the QRS complex duration, compared with controls. These results indicate that the dystrophin protein complex is required for the proper expression and function of Nav1.5. In the absence of dystrophin, decreased sodium current may explain the alterations in cardiac conduction observed in patients with dystrophinopathies.

Cardiac Voltage-Gated Sodium Channel Nav1.5 Is Regulated by Nedd4-2 Mediated Ubiquitination

Nav1.5, the cardiac isoform of the voltage-gated Na+ channel, is critical to heart excitability and conduction. However, the mechanisms regulating its expression at the cell membrane are poorly understood. The Nav1.5 C-terminus contains a PY-motif (xPPxY) that is known to act as binding site for Nedd4/Nedd4-like ubiquitin-protein ligases. Because Nedd4-2 is well expressed in the heart, we investigated its role in the ubiquitination and regulation of Nav1.5. Yeast two-hybrid and GST-pulldown experiments revealed an interaction between Nav1.5 C-terminus and Nedd4-2, which was abrogated by mutating the essential tyrosine of the PY-motif. Ubiquitination of Nav1.5 was detected in both transfected HEK cells and heart extracts. Furthermore, Nedd4-2–dependent ubiquitination of Nav1.5 was observed. To test for a functional role of Nedd4-2, patch-clamp experiments were performed on HEK cells expressing wild-type and mutant forms of both Nav1.5 and Nedd4-2. Nav1.5 current density was decreased by 65% upon Nedd4-2 cotransfection, whereas the PY-motif mutant channels were not affected. In contrast, a catalytically inactive Nedd4-2 had no effect, indicating that ubiquitination mediates this downregulation. However, Nedd4-2 did not alter the whole-cell or the single channel biophysical properties of Nav1.5. Consistent with the functional findings, localization at the cell periphery of Nav1.5-YFP fusion proteins was reduced upon Nedd4-2 coexpression. The Nedd4-1 isoform did not regulate Nav1.5, suggesting that Nedd4-2 is a specific regulator of Nav1.5. These results demonstrate that Nav1.5 can be ubiquitinated in heart tissues and that the ubiquitin-protein ligase Nedd4-2 acts on Nav1.5 by decreasing the channel density at the cell surface.

Kv4 Potassium Channels Form a Tripartite Complex With the Anchoring Protein SAP97 and CaMKII in Cardiac Myocytes

Membrane-associated guanylate kinase (MAGUK) proteins are major determinants of the organization of ion channels in the plasma membrane in various cell types. Here, we investigated the interaction between the MAGUK protein SAP97 and cardiac Kv4.2/3 channels, which account for a large part of the outward potassium current, Ito, in heart. We found that the Kv4.2 and Kv4.3 channels C termini interacted with SAP97 via a SAL amino acid sequence. SAP97 and Kv4.3 channels were colocalized in the sarcolemma of cardiomyocytes. In CHO cells, SAP97 clustered Kv4.3 channels in the plasma membrane and increased the current independently of the presence of KChIP and dipeptidyl peptidase-like protein-6. Suppression of SAP97 by using short hairpin RNA inhibited Ito in cardiac myocytes, whereas its overexpression by using an adenovirus increased Ito. Kv4.3 channels without the SAL sequence were no longer regulated by Ca2+/calmodulin kinase (CaMK)II inhibitors. In cardiac myocytes, pull-down and coimmunoprecipitation assays showed that the Kv4 channel C terminus, SAP97, and CaMKII interact together, an interaction suppressed by SAP97 silencing and enhanced by SAP97 overexpression. In HEK293 cells, SAP97 silencing reproduced the effects of CaMKII inhibition on current kinetics and suppressed Kv4/CaMKII interactions. In conclusion, SAP97 is a major partner for surface expression and CaMKII-dependent regulation of cardiac Kv4 channels.

Variable Na(v)1.5 protein expression from the wild-type allele correlates with the penetrance of cardiac conduction disease in the Scn5a(+/-) mouse model.

Background Loss-of-function mutations in SCN5A, the gene encoding Nav1.5 Na+ channel, are associated with inherited cardiac conduction defects and Brugada syndrome, which both exhibit variable phenotypic penetrance of conduction defects. We investigated the mechanisms of this heterogeneity in a mouse model with heterozygous targeted disruption of Scn5a (Scn5a +/− mice) and compared our results to those obtained in patients with loss-of-function mutations in SCN5A. Methodology/Principal Findings Based on ECG, 10-week-old Scn5a +/− mice were divided into 2 subgroups, one displaying severe ventricular conduction defects (QRS interval>18 ms) and one a mild phenotype (QRS≤18 ms; QRS in wild-type littermates: 10–18 ms). Phenotypic difference persisted with aging. At 10 weeks, the Na+ channel blocker ajmaline prolonged QRS interval similarly in both groups of Scn5a +/− mice. In contrast, in old mice (>53 weeks), ajmaline effect was larger in the severely affected subgroup. These data matched the clinical observations on patients with SCN5A loss-of-function mutations with either severe or mild conduction defects. Ventricular tachycardia developed in 5/10 old severely affected Scn5a +/− mice but not in mildly affected ones. Correspondingly, symptomatic SCN5A–mutated Brugada patients had more severe conduction defects than asymptomatic patients. Old severely affected Scn5a +/− mice but not mildly affected ones showed extensive cardiac fibrosis. Mildly affected Scn5a +/− mice had similar Nav1.5 mRNA but higher Nav1.5 protein expression, and moderately larger INa current than severely affected Scn5a +/− mice. As a consequence, action potential upstroke velocity was more decreased in severely affected Scn5a +/− mice than in mildly affected ones. Conclusions Scn5a +/− mice show similar phenotypic heterogeneity as SCN5A-mutated patients. In Scn5a +/− mice, phenotype severity correlates with wild-type Nav1.5 protein expression.

Long-Term Amiodarone Administration Remodels Expression of Ion Channel Transcripts in the Mouse Heart

Background—The basis for the unique effectiveness of long-term amiodarone treatment on cardiac arrhythmias is incompletely understood. The present study investigated the pharmacogenomic profile of amiodarone on genes encoding ion-channel subunits. Methods and Results—Adult male mice were treated for 6 weeks with vehicle or oral amiodarone at 30, 90, or 180 mg · kg−1 · d−1. Plasma and myocardial levels of amiodarone and N-desethylamiodarone increased dose-dependently, reaching therapeutic ranges observed in human. Plasma triiodothyronine levels decreased, whereas reverse triiodothyronine levels increased in amiodarone-treated animals. In ECG recordings, amiodarone dose-dependently prolonged the RR, PR, QRS, and corrected QT intervals. Specific microarrays containing probes for the complete ion-channel repertoire (IonChips) and real-time reverse transcription–polymerase chain reaction experiments demonstrated that amiodarone induced a dose-dependent remodeling in multiple ion-channel subunits. Genes encoding Na+ (SCN4A, SCN5A, SCN1B), connexin (GJA1), Ca2+ (CaCNA1C), and K+ channels (KCNA5, KCNB1, KCND2) were downregulated. In patch-clamp experiments, lower expression of K+ and Na+ channel genes was associated with decreased Ito,f, IK,slow, and INa currents. Inversely, other K+ channel &agr;- and &bgr;-subunits, such as KCNA4, KCNK1, KCNAB1, and KCNE3, were upregulated. Conclusions—Long-term amiodarone treatment induces a dose-dependent remodeling of ion-channel expression that is correlated with the cardiac electrophysiologic effects of the drug. This profile cannot be attributed solely to the amiodarone-induced cardiac hypothyroidism syndrome. Thus, in addition to the direct effect of the drug on membrane proteins, part of the therapeutic action of long-term amiodarone treatment is likely related to its effect on ion-channel transcripts.

Nedd4-2-dependent ubiquitylation and regulation of the cardiac potassium channel hERG1

The voltage-gated cardiac potassium channel hERG1 (human ether-à-gogo-related gene 1) plays a key role in the repolarization phase of the cardiac action potential (AP). Mutations in its gene, KCNH2, can lead to defects in the biosynthesis and maturation of the channel, resulting in congenital long QT syndrome (LQTS). To identify the molecular mechanisms regulating the density of hERG1 channels at the plasma membrane, we investigated channel ubiquitylation by ubiquitin ligase Nedd4-2, a post-translational regulatory mechanism previously linked to other ion channels. We found that whole-cell hERG1 currents recorded in HEK293 cells were decreased upon neural precursor cell expressed developmentally down-regulated 4-2 (Nedd4-2) co-expression. The amount of hERG1 channels in total HEK293 lysates and at the cell surface, as assessed by Western blot and biotinylation assays, respectively, were concomitantly decreased. Nedd4-2 and hERG1 interact via a PY motif located in the C-terminus of hERG1. Finally, we determined that Nedd4-2 mediates ubiquitylation of hERG1 and that deletion of this motif affects Nedd4-2-dependent regulation. These results suggest that ubiquitylation of the hERG1 protein by Nedd4-2, and its subsequent down-regulation, could represent an important mechanism for modulation of the duration of the human cardiac action potential.