The Role of CD44 and CREB in the Pathogenesis of Atrial Fibrillation

Atrial fibrillation (AF) is a common tachyarrhythmia in clinical practice. Electrical, contractile, and structural remodeling in atrial myocytes as well as fibrosis in atrial extracellular matrix contribute greatly to the pathogenesis of AF. At the cellular level, the abbreviation of action potential duration due to a decreased L-type calcium channel (LCC) in atrial myocytes and fibrotic response in atrial fibroblasts constitute the main feature of AF. Nevertheless, mechanism behind the association among decreased LCC expression, atrial fibrosis, and AF occurrence still remained unresolved. Numerous studies have linked atrial fibrosis to transforming growth factor beta (TGF-β). Furthermore, atrial tachycardia itself could alter atrial electrical properties in a way that promotes AF induction and maintenance (“AF begets AF”). Our prior study indicated that tachy-pacing in atrial-derived myocytes (HL-1 cell line) enhanced TGF-β, which in return promoted the up-regulation of collagen in atrial fibroblasts through a paracrine-dependent action. CD44 is a membrane protein controlling inflammation, cell migration, adhesion, activation, tissue repairing, and fibrosis. Recent studies demonstrated that blocking CD44 by neutralizing antibodies decrease pulmonary fibrosis in a model of pulmonary hypertension. Our preliminary results revealed that atrial fibroblasts showed a stronger response to TGF-β1 in expressing CD44 than ventricular fibroblasts either in vitro or in vivo. Furthermore, administration of the same anti-CD44 neutrolizing antibody to TGF-β1-treated atrial fibroblasts attenuated the promoting effect of TGF-β1 on collagen (key fibrotic protein) expression either in vitro or in vivo. Neutralizing anti-CD44 antibody treatment could also diminish pacing-induced AF in transgenic mice with cardiac over-expression of TGF-β. Therefore, we conclude that TGF-β shares some common signal pathways with CD44, and inhibition of CD44 might suppress atrial fibrosis and AF. At the atrial myocyte level, we found that tachy-pacing in HL-1 cells up-regulated CD44 and CREB activation, while down-regulated LCC expression. Therefore, we hypothesize that there is a direct interrelation between CD44 and CREB activation in tachy-pacing-induced LCC down-regulation. In this study, we will clarify the signaling mechanisms how CD44 affects the transcriptional activity of LCC and collagen in atrial myocytes and fibroblasts, respectively, especially focusing on the potential role in CREB. Therefore, the aim of our study will investigate whether: 1. Whether administration of TGF-β to atrial fibroblasts may induce CREB activation? 2. Whether TGF-β over-expression transgenic mice may increase CREB activation? 3. Whether the induction of collagen in atrial fibroblasts by TGF-β is dependent on CD44 and CREB pathways? 4. Whether there is an interrelation between CD44 and CREB in either atrial fibroblasts or atrial myocytes? 5. Whether neutralizing of CD44 with a monoclonal antibody will damper the TGF-β-induced CREB activation in atrial fibroblasts? 6. Whether specific blocking of downstream signals of CD44 reverses tachy-pacing-induced down-regulation of LCC in atrial myocytes? 7. Whether tachy-pacing-induced AF may decrease in TGF-β over-expression and CD44-/- mice. Hopefully, our study will provide a deeper understanding about the interaction among CD44, TGF-β, CREB and their effects on AF. The results of our study may provide a useful target for therapeutic intervention in chronic AF.

Project ID:PC10408-2061
External Project ID:MOST104-2314-B182-052-MY3