Abstract 13999: Atrial-Specific BetaM Protein is a Novel Transcriptional Regulator of Cardiac Hypertrophic Response and Myogenesis

Abstract

Background: The ability to appropriately repair or regenerate functional cardiac tissue is the major force determining adverse outcomes after cardiac injury. We previously discovered an evolutionary transformation of the Na,K-ATPase BetaM-subunit (encoded by Atp1b4 gene) into a muscle specific transcriptional regulator where it resides in the inner nuclear membrane and regulates the TGF-β signaling pathway through direct interaction with nuclear transcriptional co-regulator Ski-interacting protein (SKIP) and augmentation of inhibitory Smad7 expression. We hypothesized that BetaM is a novel transcriptional regulator of genes involved in cardiac hypertrophy and myogenesis. Methods/Results: RT-PCR and western blot analyses demonstrated that cardiac BetaM is expressed exclusively in intact rat atrial myocytes. Using luciferase reporter assays, we demonstrated that expression of exogenous BetaM in C2C12 cells up-regulates cardiac transcription factors such as myocyte enhancer factor-2 (Mef2, 2 fold), Nkx2.5 (6 fold) and atrial natriuretic factor (ANF, 2 fold). Notably, BetaM also up-regulated endogenous MyoD (a master regulator of myogenesis) mRNA and protein. Furthermore, luciferase reporter assays showed that BetaM up-regulates the MyoD reporter activity (3 fold) in a dose dependent manner. Using chromatin immunoprecipitation (ChIP) and electrophoretic mobility shift assays (EMSA) in rat neonatal skeletal muscle, BetaM was identified as a component MyoD-transcriptional complex bound to specific E-box and CArG elements of the distal regulatory region (DRR) region of the MyoD promoter. To further characterize BetaM function in vivo, transcriptome analysis by RNA Seq was performed in myocytes from newborn Atp1b4 KO and WT mice and revealed strong changes in expression of components of the Ca 2+ -Calcineurin – NFATc2 signaling cascade involved in pathological cardiac remodeling and heart failure. Conclusion: Our studies reveal that BetaM regulates the activity of essential cardiac hypertrophic response genes and master regulators of myogenesis indicating that BetaM is a hitherto unknown transcriptional regulator of cardiac hypertrophic response and myogenesis and may represent a novel therapeutic target for cardiac disease.