Resistance to Atrial Fibrillation Domestication and Mitochondrial Dysfunction in Sheep: a potential key role of the TCA Cycle and mitochondrial redox state

Abstract

BackgroundAtrial fibrillation (AF) often progresses from paroxysmal to more stable forms. It is well-recognized that patients vary in their AF progression, but underlying mechanisms remain unclear. This work, performed in a sheep AF-model, aimed to identify atrial redox and energetic status differences between animals developing stable AF (AF-S) versus those resistant to AF-stabilization (AF-R). MethodsAF was monitored with telemetry and maintained with bursts of atrial tachystimulation whenever sinus rhythm resumed. Electrophysiological remodeling was assessed via contact mapping. Structural remodeling was described by histology. Proteomic, metabolomic, enzymatic and bioenergetic remodeling were evaluated using frozen left atrial appendage (LAA) tissues and isolated LAA mitochondria. Healthy young rats were used to investigate if an induced metabolic challenge could stabilize AF episodes upon transesophageal atrial tachypacing challenge. ResultsAF-S sheep developed stable AF (>24-hours self-sustained) after 13 days on average, whereas AF-R sheep failed to develop self-sustained AF despite 120 days of electrically-maintained AF. Contact mapping and histological analysis revealed similar electro-structural remodeling in both groups. Metabolic analysis showed significant differences in tricarboxylic acid (TCA) cycle enzymes activities and a 45% increase in AF-S LAA succinate content versus AF-R. AF-S mitochondria showed abnormal mitochondrial succinate oxidation, associated with a significant 20% decrease in ATP synthesis rate, 22% increase in ROS emission and mitochondrial inner membrane hyperpolarization. The ratios of ATP to ADP, NAD+ to NADH, and Complex I/II were disturbed in AF-S compared to AF-R. Calculated mitochondrial NAD+ to NADH ratios suggest a reduced state of in-vivo AF-R mitochondria compared to the oxidized state of AF-S. Exogenous succinate was metabolized when incubated with rat atrial cardiomyocytes and altered redox balance, while intravenous succinate stabilized atrial arrhythmias induced by tachypacing in vivo. ConclusionsSheep resistant to AF-progression showed specific TCA cycle, energetic and redox adaptations compared to animals that developed self-sustained AF. In this animal model, mitochondrial TCA cycle remodeling and associated redox and energetic responses determined the resistance to AF domestication, with potential relevance to identify new mechanistic determinants of AF progression in humans.