Atrial fibrillation (AF) is promoted by various stimuli like angiotensin II, endothelin-1, adrenalin/noradrenalin and vagal stimulation (acetylcholine), all of which activate receptors coupled to G-Proteins of the Gαq/Gα11-family (Gq). Gq-mediated signal transduction has been linked to proinflammatory signaling and fibrotic remodeling in the context of cardiac pathologies. In addition, Gq-Proteins induce inositol trisphosphate (IP3) receptor-mediated intracellular Ca2+ mobilization, which is known to promote AF directly. Of note, besides being activated by neurohumoral stimuli, we recently found Gq to form a functional complex with the cation channel Piezo1, sensing mechanical stimuli and transducing them into intracellular Ca2+ mobilization and pro-inflammatory signaling. However, despite ample evidence, Gq-signaling itself has never been linked to arrhythmogenic effects or AF development.
Methods and results
In order to define the role of Gq-mediated signaling and mechanotransduction in AF, we generated transgenic mice with tamoxifen-inducible, cardiomyocyte-specific Gαq/Gα11-deficiency (Gq-KO). Gq-KO mice and control mice were exposed to intracardiac electrophysiological studies. Inducibility of AF was determined before and after carbachol-induced vagal activation using a standardized pacing protocol (Verheule et al; Fig.1). Baseline electrophysiological parameters, including heart rate, sinus node recovery time and atrial as well as AV nodal effective refractory periods were comparable in Gq/G11-KO vs. control mice, with no significant differences between the two groups before or after carbachol-induced vagal activation. However, inducibility and mean duration of AF were significantly reduced in cardiomyocyte-specific Gq -KO mice – both before and after vagel stimulation.
To explore underlying mechanisms, we investigated the role of the Gq pathway for atrial Ca2+ mobilization during excitation-contraction coupling. Left atrial cardiomyocytes were isolated from and Gq-KO mice and the respective controls. Subsequently cells were electrically stimulated and Ca2+ signaling was studied using confocal microscopy. While Ca2+ transient amplitudes, kinetics and Ca2+ content of the sarcoplasmatic reticulum (SR) were unchanged, we observed significantly fewer spontaneous pro-arrhythmic Ca2+ waves in Gq-KO compared to control mice during rest (Fig. 2). Ca2+ wave propagation velocity did not differ. However, consistent with the notion of Gq-mediated proarrhythmic signaling, SR content-corrected spontaneous Ca2+ spark frequency as well as individual spark parameters were significantly altered in Gq-KO compared to control mice. Moreover, nuclear Ca2+ transient amplitudes during excitation contraction coupling were significantly decreased in Gq-KO, indicating profound Gq-dependent differences of local Ca2+ signaling.
Conclusion
Gq-KO significantly reduced AF inducibility in-vivo, potentially related to fewer spontaneous Ca2+ waves and altered Ca2+ spark properties as indicated by our in-vitro data. Moreover, nuclear Ca2+ release was decreased in Gq-KO mice, which may be explained by the higher expression of IP3 receptors in the nuclear membrane. These data indicate a central role of Gq in the pathomechanism of AF.
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