Acute contractile activity induces the activation of the mitochondrial integrated stress response and the transcription factor ATF4

Skeletal muscle relies on mitochondria to produce energy and support its metabolic flexibility. The function of the mitochondrial pool is regulated by quality control (MQC) processes. The integrated stress response (ISR), a MQC pathway, is activated in response to various cellular stressors. The transcription factor ATF4, the main effector of the ISR, ameliorates cellular stress by upregulating protective genes, such as CHOP and ATF5. Recent literature has shown that the ISR is activated upon mitochondrial stress; however, whether this includes acute exercise-induced stress is poorly defined. To investigate this, a mouse in situ hindlimb protocol was utilized to acutely stimulate muscles at 0.25, 0.5, and 1 tetanic contraction/s for 9 min, followed by a 1-h recovery period. CAMKII alpha and JNK2 were robustly activated sixfold immediately after the protocol. ISR activation, denoted as the ratio of phosphorylated to total eIF2 alpha protein levels, was also elevated after recovery. Downstream, contractile activity induced an increase in the nuclear localization of ATF4. Robust twofold increases in the mRNA expression of ATF4 and CHOP were also observed after the recovery period. Changes in ATF4 mRNA were independent of transcriptional activation, as assessed with an ATF4 promoter-reporter plasmid. Instead, mRNA decay assays revealed an increase in ATF4 mRNA stability post contractile activity, as a result of enhanced stabilization by the RNA binding protein HuR. Thus, acute contractile activity is sufficient to induce mitochondrial stress and activate the ISR, corresponding to the induction of ATF4 with potential consequences for mitochondrial phenotype adaptations in response to repeated exercise. NEW & NOTEWORTHY The integrated stress response (ISR) is a mitohormetic stress response critical for the maintenance of mitochondrial homeostasis. However, its role in mediating mitochondrial adaptations with exercise-induced stress is not well established. This research demonstrates that acute contractile activity can elicit mitochondrial stress and activate the ISR to maintain mitochondrial homeostasis via the enhancement of the functioning of ATF4, illustrating an early response to exercise that promotes mitochondrial health and adaptations.