Abstract-The cardiac sodium channel Na v 1.5 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 Na v 1.5 C-terminus. Pull-down experiments were performed using Na v 1.5 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 mdx 5cv mice to study the role of this protein complex in Na v 1.5 function. Western blot experiments revealed a 50% decrease in the Na v 1.5 protein levels in mdx 5cv hearts, whereas Na v 1.5 mRNA levels were unchanged. Patch-clamp experiments showed a 29% decrease of sodium current in isolated mdx 5cv cardiomyocytes. Finally, ECG measurements of the mdx 5cv 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 Na v 1.5. In the absence of dystrophin, decreased sodium current may explain the alterations in cardiac conduction observed in patients with dystrophinopathies. Key Words: Duchenne dystrophy Ⅲ dystrophin Ⅲ ECG Ⅲ mouse Ⅲ sodium channels Ⅲ syntrophin T he main cardiac voltage-gated sodium channel, Na v 1.5, generates the fast depolarization of the cardiac action potential, and plays a key role in cardiac conduction. Its importance for normal cardiac function has been exemplified by the description of numerous naturally occurring genetic variants of the gene SCN5A, which encodes Na v 1.5, that are linked to various cardiac diseases. 1 Among them, the congenital long QT syndrome type-3 and the Brugada syndrome are caused by gain or loss-of-function of Na v 1.5, respectively. 1 Na v 1.5 is the pore-forming ␣-subunit protein of the cardiac sodium channel. It has a molecular weight of Ϸ220 kDa, and may be associated with at least 4 types of auxiliary small (30 to 35 kDa) -subunits. Recently, several proteins that bind directly to Na v 1.5 have been described. 2 However, in most cases the physiological relevance of these interactions remains poorly understood, mainly because of a lack of appropriate animal models. With the exception of ankyrin-G, 3 all these partner proteins interact with the 243-residues-long intracellular C-terminal domain of the channel which contains several protein-protein interaction motifs. 2 Among them, the last three residues of Na v 1.5 (2014-Ser-Ile-Val-2016) constitute a PDZ domain-binding motif to which syntrophins and dystrophin have been shown to interact directly or indirectly, respectively. 4 -6 However, thus far, the role of these interacting proteins in the heart has never been investigated.In this study, by performing mass spe...
Hemodynamic overload of the atria is an important pathogenic factor of fibrosis; MMP-7 appears to be involved in the early stage of this tissue remodeling process.
Abstract-Downregulation of the L-type Ca 2ϩ current (I Ca ) is an important determinant of the electrical remodeling of diseased atria. Using a rat model of heart failure (HF) due to ischemic cardiopathy, we studied I Ca in isolated left atrial myocytes with the whole-cell patch-clamp technique and biochemical assays. I Ca density was markedly reduced (1.7Ϯ0.1 pA/pF) compared with sham-operated rats (S) (4.1Ϯ0.2 pA/pF), but its gating properties were unchanged. Calcium channel ␣ 1C -subunit quantities were not significantly different between S and HF. The -adrenergic agonist isoproterenol (1 mol/L) had far greater stimulatory effects on I Ca in HF than in S (2.5-versus 1-fold), thereby suppressing the difference in current density. Dialyzing cells with 100 mol/L cAMP or pretreating them with the phosphatase inhibitor okadaic acid also increased I Ca and suppressed the difference in density between S and HF. Intracellular cAMP content was reduced more in HF than in S. The phosphodiesterase inhibitor 3-isobutyl-1-methyl-xanthine had a greater effect on I Ca in HF than in S (76.0Ϯ11.2% versus 15.8Ϯ21.2%), whereas the inhibitory effect of atrial natriuretic peptide on I Ca was more important in S than in HF (54.1Ϯ4.8% versus 24.3Ϯ8.8%). Cyclic GMP extruded from HF myocytes was enhanced compared with S (55.8Ϯ8.0 versus 6.2Ϯ4.0 pmol · mL Ϫ1 ). Thus, I Ca downregulation in atrial myocytes from rats with heart failure is caused by changes in basal cAMP-dependent regulation of the current and is associated with increased response to catecholamines.
In human atrial myocytes, the variability of ICaL is related to the clinical history of the donors. The downregulation of ICaL is already observed in patients in sinus rhythm with a high risk of AF and is associated with the greatest response to beta-adrenergic agonist.
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