Maintenance of the mitochondrial protein folding environment is essential for organellar and cellular homeostasis. Over 99% of mitochondrial proteins require import into mitochondria, followed by their folding and intraorganellar sorting. Mitochondrial stress can result in the accretion of misfolded proteins, establishing a requirement for mitochondrial protein quality control (MQC) strategies. The Mitochondrial Unfolded Protein Response (UPRmt) is a compartment-specific MQC mechanism that increases the expression of protective enzymes by Activating Transcription Factor 5 (ATF5) to restore mitochondrial function. Contractile activity during acute exercise is a stressor that has the potential to temporarily disrupt organellar protein homeostasis. However, the roles of ATF5 and the UPRmt in basal mitochondrial maintenance and exercise-induced UPRmt signaling in skeletal muscle are not known. To investigate this, we subjected WT and whole-body ATF5 KO mice to a bout of acute exercise and collected skeletal muscle tissues immediately after. ATF5 KO animals exhibited 2-fold increases in phosphorylated JNK protein levels, indicative of enhanced stress signaling. Interestingly, in KO muscle, PGC-1a protein was enhanced by 50% and 40% in nuclear and cytosolic compartments, respectively, suggesting an increased drive toward mitochondrial biogenesis in the absence of ATF5. Muscle from these animals also displayed a more abundant, but dysfunctional, mitochondrial pool, with a 20% increase in mitochondrial content, 30-40% reductions in respiration, and a 20% increase in ROS emissions, corresponding with no changes in exercise performance. The UPRmt proteins mtHSP70 and LONP were upregulated 20-30% in KO muscle, while ATF4 mRNA was upregulated 2.5-3.7-fold, along with an 8% increase in its nuclear localization. Furthermore, KO muscle showed an impaired UPRmt mRNA response to acute exercise, suggesting a regulatory role for ATF5 in the maintenance of a high-quality mitochondrial pool, and in mediating the transcription of UPRmt genes during exercise.
