Mechanical regulation of the glucocorticoid-sensitive transcriptome in young and aged skeletal muscle

Elevated glucocorticoids alter the skeletal muscle transcriptome to induce a myopathy characterized by muscle atrophy, muscle weakness, and decreased metabolic function. These effects are more likely to occur and be more severe in aged muscle. Resistance exercise can blunt development of glucocorticoid myopathy in young muscle, but the potential to blunt the signals initiating myopathy in aged muscle is unknown. To answer this, young (4-month-old) and aged (24-25-month-old) male C57BL/6 mice were randomized to receive either an intraperitoneal (IP) injection of dexamethasone (DEX; 2 mg/kg) or saline as a control. Two hours post-injections, tibialis anterior (TA) muscles of mice were subjected to unilateral high force contractions. Muscles were harvested four hours later. The glucocorticoid- and contraction-sensitive genes were determined by RNA sequencing. The number of glucocorticoid sensitive genes was similar between young and aged muscle. Contractions altered more glucocorticoid-sensitive genes in aged muscle, with this outcome primarily occurring when hormone levels were elevated. Glucocorticoid-sensitive gene programs altered by contractions were primarily related to metabolism in young mice and muscle size regulation and inflammation in aged mice. In silico analysis implied Peroxisome proliferator-activated receptor gamma-1 (PPARG) contributed to the contraction-induced changes in glucocorticoid-sensitive genes in aged muscle. Increasing PPARG expression in the TA of aged mice using Adeno-associated virus serotype 9 partially counteracted the glucocorticoid-induced reduction in Runt-related transcription factor 1 (Runx1) mRNA content, recapitulating the effects observed by contractions. Overall, these data contribute to our understanding of the mechanical regulation of the glucocorticoid transcriptome in aged skeletal muscle. Overall design: 4-mo-old male mice (Young) and 24-25-mo-old male mice (Aged) were randomized to receive an intraperitoneal (IP) injection of dexamethasone at 2 mg/kg (DEX) diluted in saline or an IP injection of saline only as a control (Vehicle). The mice were given 2 h recovery after the injection to allow for full activation of the glucocorticoid receptor in the DEX treated group. Mice were then subjected to unilateral high force eccentric contractions by electrical stimulation of the sciatic nerve under deep isoflurane (3%) anesthesia. Following the contractions, all mice received a 500 µL subcutaneous injection of warm saline before returning to their cages. During the recovery, all mice had access to water but not food. Four hours after completing the contractions protocol (6.5 h post-DEX or saline injection), both tibialis anterior (TA) muscles (Non-Stim and Stim leg) were extracted under deep isoflurane (3%) anesthesia, snap-frozen in liquid nitrogen, and stored at -80°C. Total RNA extracted from the TA muscles generated was subjected to RNA sequencing to determine acute glucocortocoid- and contraction-sensitive genes (Young-Vehicle: n=6 mice; Young-DEX: n=6 mice; Aged-Vehicle: n=4 mice; Aged-DEX: n=6 mice).