Is ATF4 Required for Skeletal Muscle Mitochondrial Biogenesis and Remodeling?

Skeletal muscle is an important tissue for the maintenance of whole‐body health and vitality. Regarded for its role in supporting posture and locomotion, the metabolic profile of muscle has further ramifications for mobility, risk of falls/injury, and the development of diseases. As mitochondria are responsible for the maintenance of metabolic health in muscle, they also contribute to muscle dysfunction and disease. The intricate regulation of mitochondrial content and function within muscle is essential to match the internal metabolic capability to the external demands being placed on the tissue. PGC‐1α is the master regulator of mitochondrial biogenesis, which refers to the synthesis of mitochondrial proteins transcribed by both nDNA and mtDNA. The synchronous expression of mitochondrial proteins from either genome is important in preserving proteostasis, as an imbalance in their expression can promote dysfunction within the organelle and trigger the mitochondrial unfolded protein response (UPR mt ). ATF4, identified as a primary regulator of the UPR mt , upregulates the expression of mitochondrial chaperones and proteostasis to augment the protein handling ability of the organelle, and to re‐establish homeostasis. As ATF4 responds to acute cellular stresses, it has been implicated in regulating skeletal muscle health by mediating aging‐related, and disuse‐induced muscle atrophy and decline. However, while it is understood that ATF4 is involved in mediating the mitochondrial stress response, it has yet to be determined whether ATF4 is necessary for mitochondrial biogenesis in skeletal muscle. We measured ATF4 expression in C2C12 cells prior to, and following 4‐days of differentiation, and found ATF4 protein levels to be elevated over 3‐fold in differentiated myotubes. This induction of ATF4 coincided with 3–5‐fold increases in protein expression of mitochondrial content markers COX I and IV, despite modest decreases in UPR mt factors, mtHSP70, HSP60, and CPN10. Moreover, overexpression and knockdown of ATF4 in cultured myoblasts affected their ability to form multinucleated myotubes, indicating that the expression of ATF4 must be regulated to facilitate myotube formation. We examined the induction of ATF4 following contractile activity (CA), both in C2C12 myotubes, and rat TA muscle. CA was sufficient to induce 50–80% increases in ATF4 mRNA and protein in tissue and cultured cells, respectively, which preceded 1.5–2.5‐fold increases in mitochondrial content. Additionally, we observed a 2‐fold increase in ATF4 protein expression in hindlimbs of mice subjected to unilateral sciatic denervation, which corresponded with a 15–20% decrease in mitochondrial content. Taken together, these data suggest a possible regulatory role for ATF4 in determining mitochondrial adaptation within muscle during development, exercise and chronic disuse. Subsequent work investigating the impact of ATF4 ablation and exogenous expression on mitochondrial biogenesis signaling, as well as on mitochondrial function following contractile activity will further illustrate whether ATF4 is necessary and sufficient to promote improvements in mitochondrial content and function. Support or Funding Information Work supported by NSERC, Canada.

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