Role of the ATP-dependent chromatin-remodeling enzyme Brg1 in the regulation of cardiac Na+ channel

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NaV1.5, encoded by SCN5A gene, is a main α subunit of the cardiac Na+ channel. Na+ channel activity determines cardiac excitability and electrical conduction. Decreased Na+ channel activity induced by suppression of NaV1.5 expression is linked to ventricular tachycardia and fibrillation (VT/VF) in ischemic heart disease (IHD), but the mechanisms of the NaV1.5 downregulation are largely unknown. Chromatin remodeling by Brahma- related gene-1 (BRG1), a central catalytic subunit of numerous chromatin-modifying enzymatic complexes, play an essential role in cardiac development and dysfunction by reprogramming gene expression under pathophysiological conditions. BRG1 remodels chromatin activity and facilitates a subset of genes via interaction with sequence-specific transcription factors. Our preliminary results showed that 1. NaV1.5 expression is decreased in human hearts with IHD and mouse hearts with myocardial infarction (MI), 2. Na+ channel activity is decreased in the peri-infarct zone (PIZ) of mouse MI hearts, 3. BRG1 is increased with BRG1 and β-catenin nuclear accumulation of cardiomyocytes in human IHD and PIZ of mouse MI hearts, 4. Reactive oxygen species (ROS) elevated in IHD increases BRG1 expression and decreases NaV1.5 expression by enhancing β- catenin/TCF4 signaling, 5. A complex of BRG1/β-catenin/TCF4 complex recruited in TCF4 binding site inhibits SCN5A promoter in HL-cells and suppresses NaV1.5 expression and Na+ channel activity, 6. BRG1 was detected in the nuclei of adult cardiomyocytes; however, cardiac-specific knockout (KO) of BRG1 did not affect NaV1.5 expression and Na+ channel activity. Interestingly, enhanced β-catenin/TCF4 by cardiac-specific deletion of β- catenin exon 3 (β-cat∆E3) suppressed NaV1.5 expression and Na+ channel activity, leading to susceptibility to VT in mice challenging with flecainide (Ic antiarrhythmic drug) which was prevented by BRG1 KO. Immunoprecipitation showed BRG1 interacts with β-cat∆E3 rather than β-catenin in adult cardiomyocytes, suggesting BRG1 suppressing NaV1.5 expression is dependent on the enhanced β-catenin/TCF4 signaling. These preliminary findings support our hypothesis that BRG1 interacts with β-catenin to promote β-catenin/TCF4 signaling-mediated suppression of NaV1.5, leading to deceleration of cardiac depolarization and development of VT/VF in IHD. We will test this hypothesis in 2 specific aims, AIM 1: To determine whether increased BRG1 is necessary for enhanced β-catenin/TCF4 signaling-mediated suppression of NaV1.5 expression, leading to decreased Na+ channel activity in moue MI hearts; AIM 2: To determine whether BRG1 suppressing NaV1.5 expression and Na+ channel activity by inhibiting SCN5A promoter activity through β-catenin/TCF4. In order to achieve these 2 specific AIMs, we will use transgenic mice with cardiac BRG1 KO, β-cat∆E3, BRG1 KO/β-cat∆E3 and TCF4 KO, BRG1 OE, and BRG1 OE/TCF4 KO in combination of MI procedure and in vitro studies to determine whether BRG1 facilitates β-catenin/TCF4 suppression of NaV1.5 expression, leading to decreased Na+ channel activity and VT in mice with MI.