Delayed AV-conduction in pressure-overloaded Scn5a-1798insD+/- mice rescued by genetic deletion of the calcium-dependent transcription factor Nfatc2

Introduction: Transgenic Scn5a-1798insD+/- mice display atrio-ventricular conduction disturbances and develop susceptibility to arrhythmias with age. Isolated cardiomyocytes from Scn5a-1798insD+/- mouse hearts exhibit increased sustained inward sodium current which is associated with elevated intracellular sodium (Na+), intracellular diastolic calcium ([Ca2+]i) levels and Ca2+ transient amplitudes. We investigated whether the Scn5a-1798insD+/- mutation leads to activation of Ca2+-dependent calcineurin/Nfat signaling pathway, and assessed its impact on atrio-ventricular conduction. Results: Despite increased [Ca2+]i, activation of the calcineurin/Nfat pathway was not observed in Scn5a-1798insD+/- hearts at baseline (as evidenced by unaltered nuclear levels of Nfat protein as compared to wild type hearts). Wild-type and Scn5a-1798insD+/- mice displayed a similar hypertrophic response to 2 weeks of TAC (as evidenced by similar increases in heart weight and Anf mRNA levels), indicating equal activation of the calcineurin/Nfat pathway. However, Scn5a-1798insD+/–TAC mice displayed increased incidence of sudden death preceded by progressive bradycardia and AV-block. Furthermore, isolated Scn5a-1798insD+/–TAC hearts showed AV-block and significantly prolonged AV-delay as compared to wild-type-TAC hearts. Since Nfatc2 is a well-described downstream effector of the calcineurin pathway, we next investigated whether the observed electrical abnormalities after TAC could be rescued by crossing Scn5a-1798insD+/- with Nfatc2-KO mice. Scn5a-1798insD+/-/Nfatc2-KO mice showed a lower response to TAC as Scn5a-1798insD+/–TAC mice (as evidenced by lower increase in heart weight and Anf mRNA levels). The ex vivo AV-delay after TAC was attenuated in Scn5a-1798insD+/-/Nfatc2-KO TAC hearts, indicating rescue of the AV-conduction disturbances in Scn5a-1798insD+/–TAC mice by Nfatc2 deficiency. Moreover, His-ventricular conduction was unaffected by TAC, indicating that the increase in AV-delay in Scn5a-1798insD+/–TAC mice occurs proximal to the His bundle. Conclusions: Scn5a-1798insD+/- mice show intracellular Na+/Ca2+-dysregulation at baseline and severe AV-conduction abnormalities and sudden death upon increased activation of the calcineurin/Nfat-pathway by TAC challenge. Nfatc2 deficiency prevents these electrical disturbances secondary to TAC in Scn5a-1798insD+/-/Nfatc2-KO mice, indicating a role of the calcineurin/Nfat pathway on atrio-ventricular conduction.