Skeletal Muscle Mitochondria Contain Nuclear-Encoded RNA Species Prior to and Following Adaptation to Exercise Training in Rats

Skeletal muscle mitochondria adaptation to exercise training is mediated by molecular factors that are not fully understood. Mitochondria import over 1000 proteins encoded by the nuclear genome, but the RNA population resident within the organelle is generally thought to be exclusively encoded by the mitochondrial genome. However, recent in vitro evidence suggests that specific nuclear-encoded miRNAs and other noncoding RNAs (ncRNAs) can reside within the mitochondrial matrix. Whether these are present in mitochondria of skeletal muscle tissue, and whether this is affected by endurance training-a potent metabolic stimulus for mitochondrial adaptation-remains unknown. Rats underwent 4 weeks of moderate-intensity treadmill exercise training, then were humanely killed and tissues were collected for molecular profiling. Mitochondria from gastrocnemius skeletal muscle were isolated by immunoprecipitation, further purified, and then the resident RNA was sequenced to assess the mitochondrial transcriptome. Exercise training elicited typical transcriptomic responses and functional adaptations in skeletal muscle, including increased mitochondrial respiratory capacity. We identified 24 nuclear-encoded coding or noncoding RNAs in purified mitochondria, in addition to 50 nuclear-encoded miRNAs that met a specified abundance threshold. Although none were differentially expressed in the exercise vs. control group at FDR < 0.05, exploratory analyses suggested that the abundance of 3 miRNAs was altered (p < 0.05) in mitochondria isolated from trained compared with sedentary skeletal muscle. We report the presence of a specific population of nuclear-encoded RNAs in the mitochondria isolated from rat skeletal muscle tissue, which could play a role in regulating exercise adaptations and mitochondrial biology.