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Alternative splicing is a posttranscriptional mechanism that can substan-tially change the pattern of gene expression. Up to 95% of human genes have multiexon alternative spliced forms, suggesting that alternative splicing is one of the most significant components of the functional complexity of the human genome. Nevertheless, alternative splicing regulation has received comparatively little attention in the study of cardiac diseases. When investigating SCN5A splicing abnormalities in heart failure (HF), we found that 47 of 181 known splicing regulators were upregulated in HF compared to controls, which indicates that splicing regulation may play a key role in HF. Our results show that angiotensin II and hypoxia, signals common to HF, result in increased LUC7L3 and RBM25 splicing regulators, increased binding of RBM25 to SCN5A mRNA, increased SCN5A splice variant abundances, decreased full-length SCN5A mRNA and protein, and decreased Na+ current. These observations may shed light on a mechanism whereby cardiac function and arrhythmic risk are associated and allow for refined predictions of which patients may be at highest arrhythmic risk or suffer from Na+ channel blocking anti-arrhythmic drug complications. [ABSTRACT FROM AUTHOR]

The article presents a study which examined the involvement of nicotinamide adenine dinucleotide (NADH) in the regulation of human cardiac sodium channels (NaV1.5). The study discussed the association of NaV1.5 mutations with Brugada syndrome (BrS.) The researchers used rate neonatal cardiomyocytes and HEK293 cells. The study revealed that pyridine nucleotides regulate NaV1.5, and suggested connection between NAD-dependent energy metabolism with sodium current.

The SCN5A gene encodes a voltage-sensitive sodium channel expressed in cardiac and skeletal muscle. Coding region mutations cause cardiac sudden death syndromes and conduction system failure. Polymorphisms in the 5'-sequence adjacent to the SCN5A gene have been linked to cardiac arrhythmias. We identified three alternative 5'-splice variants (1A, 1B, and 1C) of the untranslated exon 1 and two 3'-variants in the murine Scn5a mRNA. Two of the exon 1 isoforms (1B and 1C) were novel when compared with the published human and rat SCN5A sequences. Quantitative real time PCR results showed that the abundance of the isoforms varied during cardiac development. The 1A, 1B, and 1C mRNA splice variants increased 7.8 ± 1.7-fold (E1A), 6.0 ± 1.0-fold (E1B), and 20.6 ± 3.7-fold (E1C) from fetal to adult heart, respectively. Promoter deletion and luciferase reporter gene analysis using cardiac and skeletal muscle cell lines demonstrated a pattern of distinct cardiac-specific enhancer elements associated with exons 1A and 1C. In the case of exon 1C, the enhancer element appeared to be within the exon. A 5'-repressor preceded each cardiac enhancer element. We concluded that the murine Na+ channel has both 5'- and 3'-untranslated region mRNA variants that are developmentally regulated and that the promoter region contains two distinct cardiac-specific enhancer regions. The presence of homologous human splicing suggests that that these regions may be fruitful new areas of study in understanding cardiac sodium channel regulation and the genetic susceptibility to sudden death. [ABSTRACT FROM AUTHOR]

Bufonid anurans of the genus Atelopus contain both steroidal bufadienolides and various guanidinium alkaloids of the tetrodotoxin class. The former inhibit sodium-potassium ATPases, whereas the latter block voltage-dependent sodium channels. The structure of one guanidinium alkaloid, zetekitoxin AB, has remained a mystery for over 30 years. The structure of this alkaloid now has been investigated with a sample of ≈0.3 mg, purified from extracts obtained decades ago from the Panamanian golden frog Atelopus zeteki. Detailed NMR and mass spectral analyses have provided the structure and relative stereochemistry of zetekitoxin AB and have revealed that it is an analog of saxitoxin. The proposed structure is characterized by richness of heteroatoms (C16H25N8O12S) and contains a unique 1,2-oxazolidine ring-fused lactam, a sulfate ester, and an N-hydroxycarbamate moiety. Zetekitoxin AB proved to be an extremely potent blocker of voltage-dependent sodium channels expressed in Xenopus oocytes. The IC50 values were 280 pM for human heart channels, 6.1 pM for rat brain IIa channels, and 65 pM for rat skeletal muscle channels, thus being roughly 580-, 160-, and 63-fold more potent at these channels than saxitoxin. [ABSTRACT FROM AUTHOR]

Presents information on a study which probed the rat adult skeletal muscle sodium+ channel outer vestibule and selectivity filter using mu conotoxin GIIIA. Methodology of the study; Results and discussion on the study.

Determines unitary conductances of native Na+ channel isoforms (alphaNa) under a variety of conditions. Basis of a linear least-squares fit; Comparison of nerve and cardiac Na+ channels; Mean open time; Unitary current-voltage relationships for neuroblastoma.

Mutations of the cardiac sodium channel (Nav 1.5) can induce gain or loss of channel function. Gain-of-function mutations can cause long QT syndrome type 3 and possibly atrial fibrillation, whereas loss-of-function mutations are associated with a variety of phenotypes, such as Brugada syndrome, cardiac conduction disease, sick sinus syndrome, and possibly dilated cardiomyopathy. The phenotypes produced by Nav1.5 mutations vary according to the direct effect of the mutation on channel biophysics, but also with age, sex, body temperature, and between regions of the heart. This phenotypic variability makes genotype-phenotype correlations difficult. In this Perspectives article, we propose th at phenotypic variability not ascribed to mutation-dependent changes in channel function might be the result of additional modifiers of channel behaviour, such as other genetic variation and alterations in transcription, RNA processing, translation, post-translational modifications, and protein degradation. Consideration of these modifiers might help to improve genotype-phenotype correlations and lead to new therapeutic strategies. [ABSTRACT FROM AUTHOR]

The QT interval is a recording of cardiac electrical activity. Previous genome-wide association studies identified genetic variants that modify the QT interval upstream of LITAF (lipopolysaccharide- induced tumor necrosis factor-a factor), a protein encoding a regulator of endosomal trafficking. However, it was not clear how LITAF might impact cardiac excitation. We investigated the effect of LITAF on the voltage-gated sodium channel Nav1.5, which is critical for cardiac depolarization. We show that overexpressed LITAF resulted in a significant increase in the density of Nav1.5-generated voltage-gated sodium current INa and Nav1.5 surface protein levels in rabbit cardiomyocytes and in HEK cells stably expressing Nav1.5. Proximity ligation assays showed co-localization of endogenous LITAF and Nav1.5 in cardiomyocytes, whereas co-immunoprecipitations confirmed they are in the same complex when overexpressed inHEK cells. In vitro data suggest that LITAF interacts with the ubiquitin ligase NEDD4-2, a regulator of Nav1.5. LITAF overexpression downregulated NEDD4-2 in cardiomyocytes and HEK cells. In HEK cells, LITAF increased ubiquitination and proteasomal degradation of co-expressedNEDD4-2 and significantly blunted the negative effect of NEDD4-2 on INa. We conclude that LITAF controls cardiac excitability by promoting degradation ofNEDD4-2, which is essential for removal of surface Nav1.5. LITAF-knockout zebrafish showed increased variation in and a nonsignificant 15% prolongation of action potential duration. Computer simulations using a rabbit-cardiomyocyte model demonstrated that changes in Ca21 and Na1 homeostasis are responsible for the surprisingly modest action potential duration shortening. These computational data thus corroborate findings from several genome-wide association studies that associated LITAF with QT interval variation. [ABSTRACT FROM AUTHOR]

Background—Human heart failure is associated with decreased cardiac voltage-gated Na+ channel current (encoded by SCN5A), and the changes have been implicated in the increased risk of sudden death in heart failure. Nevertheless, the mechanism of SCN5A downregulation is unclear. A number of human diseases are associated with alternative mRNA splicing, which has received comparatively little attention in the study of cardiac disease. Splicing factor expression profiles during human heart failure and a specific splicing pathway for SCN5A regulation were explored in this study. Methods and Results—Gene array comparisons between normal human and heart failure tissues demonstrated that 17 splicing factors, associated with all major spliceosome components, were upregulated. Two of these splicing factors, RBM25 and LUC7L3, were elevated in human heart failure tissue and mediated truncation of SCN5A mRNA in both Jurkat cells and human embryonic stem cell-derived cardiomyocytes. RBM25/LUC7L3-mediated abnormal SCN5A mRNA splicing reduced Na+ channel current 91.1±9.3% to a range known to cause sudden death. Overexpression of either splicing factor resulted in an increase in truncated mRNA and a concomitant decrease in the full-length SCN5A transcript. Conclusions—Of the 17 mRNA splicing factors upregulated in heart failure, RBM25 and LUC7L3 were sufficient to explain the increase in truncated forms and the reduction in full-length Na+ channel transcript. Because the reduction in channels was in the range known to be associated with sudden death, interruption of this abnormal mRNA processing may reduce arrhythmic risk in heart failure. [ABSTRACT FROM AUTHOR]