MicroRNA-135a regulates sodium-calcium exchanger gene expression and cardiac electrical activity.

BACKGROUND: Complete atrioventricular block (CAVB) causes arrhythmogenic remodeling and increases the risk of torsades de pointes arrhythmias. MicroRNAs (miRNAs) are key regulators of gene expression that contribute to cardiac remodeling.

OBJECTIVE: The purpose of this study was to assess miRNA changes after CAVB and identify novel candidates potentially involved in arrhythmogenic cardiac remodeling.

METHODS: CAVB was induced in mice via His-bundle ablation. Expression of miRNAs was evaluated by pan-miRNA microarray with quantitative polymerase chain reaction (qPCR) confirmation, on samples obtained 24 hours and 4 weeks post-CAVB. MiRNA target prediction algorithms were used to identify potential target genes. Targets confirmed by luciferase assays in HEK293 cells were followed up with overexpression studies in neonatal rat ventricular myocytes to evaluate regulation using real time- quantitative polymerase chain reaction (RT-qPCR), western blots, cell shortening measurements, and fura-2 Ca2+ fluorescence imaging.

RESULTS: Of >400 miRNAs assayed, only miRNA-135a (miR-135a) was altered at 24 hours, down-regulated 78% (P <.001). Algorithms predicted miR-135a regulation of the sodium-calcium exchanger type 1 (NCX1). miR-135a transfection suppressed NCX1 3'UTR reporter activity by 42% (P <.001), mRNA expression by 34% (P <.001), and protein levels by 45% (P <.001) vs noncoding miRNA control. miR-135a overexpression reduced spontaneous beating frequency of neonatal rat ventricular myocytes by 63% (P <.001) while slowing decay (by 56%, P <.05) of caffeine-induced Ca2+ transients. miR-135a also suppressed the Ca2+ loading effects of ouabain and ouabain-induced spontaneous Ca2+ release events.

CONCLUSION: NCX1 is negatively regulated by miR-135a, a microRNA that is down-regulated in the heart after CAVB in mice. By controlling NCX1 expression, miR-135a modulates cardiomyocyte automaticity, Ca2+ extrusion, and arrhythmogenic Ca2+ loading/spontaneous Ca2+ release events. Therefore, miR-135a may contribute to proarrhythmic remodeling after CAVB.