PROJECT SUMMARY A widely conserved hallmark of aging is the decline in muscle mass and strength, promoting frailty, loss of independence and disability. Crucial for this is our gap in knowledge regarding mechanisms linking bioenergetic stimulation to muscle anabolism. Glucocorticoid steroids have pervasive effects on energy metabolism and muscle function. Dosing intermittence appears crucial to the benefits/risks ratio of these drugs in dystrophic animal models and patients, i.e. in genetic myopathies. Here we investigate whether intermittent glucocorticoids increase performance in aged muscle, where weakness and sarcopenia are not dependent on specific genetic insults. Our new results in 24 months-old WT mice show that a chronic regimen of intermittent once-weekly prednisone increased muscle performance in aging mice to levels comparable to young adult mice (4 months- old). Remarkably, treatment rescued both mitochondrial respiration and muscle mass in aging mice to young- like levels. Mechanistically, we found that bioenergetic, functional and anabolic effects of intermittent prednisone were blunted upon inducible ablation of PGC1a in adult muscle. While the role of PGC1a in boosting mitochondrial capacity in aged muscle is more established, its effects on age-related sarcopenia and weakness are still debated with conflicting results from constitutive knockout or overexpression models. Scattered reports have hinted at a role of PGC1a in activating growth pathways, including biosynthesis of amino acids like alanine, but this is still largely unknown in muscle aging. Here we will investigate the mechanisms through which a bioenergetic stimulation like intermittent prednisone rescues both oxidative metabolism and mass gain in muscle aging. In Aim 1, we will determine the extent to which intermittence discriminates deleterious versus beneficial effects of glucocorticoids in aging muscle. We hypothesize that dosing intermittence shifts exogenous glucocorticoid effects from a PGC1a-lowering pro-wasting program to a PGC1a-dependent pro-ergogenic program, i.e. balanced gain of performance and mass. We will test this through inducible muscle-specific PGC1a ablation after natural aging. In Aim 2 we will establish the role of muscle Lipin1 in PGC1a re-activation in muscle aging. We hypothesize that Lipin1 supports mitochondrial function in muscle aging and mediates the PGC1a re- activation in response to intermittent glucocorticoids. To test this, we will use our newly derived mice with muscle- restricted inducible Lipin1 ablation after natural aging. In Aim 3, we will elucidate the extent to which bioenergetics rescue sarcopenia in aging through amino acid biosynthesis. We hypothesize that mitochondrial reactivation fuels amino acid biogenesis, supporting protein synthesis during muscle aging. We will test this by tracing glucose-derived carbons into amino acids in oxidative versus glycolytic myofibers. We will also test the extent to which muscle PGC1a...