Abstract MP113: Protein Hyperacetylation Mediates Bradyarrhythmia in Mice With Mitochondrial Disease

Background: Mice with germline mutation of the mitochondrial respiratory complex I subunit Ndufs4 exhibit progressive neurodegenerative phenotypes resembling Leigh syndrome (LS). Cardiomyocyte-specific Ndufs4 -/- mice demonstrate protein hyperacetylation and accelerated heart failure in response to pressure overload. Methods and Results: LS mice developed bradyarrhythmia with heart rate <400 bpm. EKG showed sick sinus syndrome (SSS) and high degree AV block. Supplementation of Nicotinamide Riboside (NR), which increased cellular NAD + , significantly reduced arrhythmia events and restored normal sinus rhythm, as shown by confocal scanning of sinoatrial nodal tissue dissected from LS mouse hearts. Immunoprecipitation and immunohistochemistry using antibody specific for K1479 of Nav 1.5 showed hyperacetylation of LS SA nodal tissue, which was attenuated by NR. HEK293-cells deficient in mitochondrial complex I subunit Ndufs2 had higher DCFDA (ROS) and lower TMRM intensity (mitochondrial membrane potential) compared with Ndufs4 knock-out cells. Ndufs2-knock-out HEK293 cells transfected with wild-type Nav 1.5 showed hyperacetylation of K1479 of Nav 1.5 . Patch clamp showed significantly reduced sodium current (I Na ) in Ndufs2 deficient HEK293 cells. NR prevented the decrease of I Na in Ndufs2 KO cells. HEK293 cells expressing mutant Nav 1.5 constructs mimicking deacetylation (K1479R) had increased I Na compared with HEK293 expressing WT Nav 1.5, suggesting that acetylation status affect Nav1.5 function. Arrythmia was noted in conduction tissue specific HCN4-Ndufs4 -/- mice, suggested a cell autonomous effect leading to arrhythmia. These findings indicate that mitochondrial complex I deficiency lead to hyperacetylation of Nav 1.5 , reduction of I Na , which subsequently cause bradyarrhythmia and this can be reversed by NR. Using echocardiography, we demonstrated significant diastolic dysfunction (decreased E’/A’, prolonged isovolemic relaxation time) in LS mice, which was ameliorated by NR. The calcium transient amplitude and decay rate, an indicator of ryanodine receptor and SERCA2 functions, was significantly decreased by ~15% in LS hearts and this was mitigated by NR, concomitant with reversal of SERCA2 hyperacetylation. Conclusions: Despite normal cardiac structure and systolic function, LS mice showed SSS, high degree AV block and diastolic dysfunction, which is mediated by hyperacetylation of Nav 1.5 and SERCA2, respectively. NR significantly reversed both arrhythmia and diastolic dysfunction. Our study demonstrates novel mechanistic link between mitochondrial complex I deficiency and the function of Nav 1.5 and SERCA2.