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Aging and many illnesses and injuries impair skeletal muscle mass and function, but the molecular mechanisms are not well understood. To better understand the mechanisms, we generated and studied transgenic mice with skeletal muscle-specific expression of growth arrest and DNA damage inducible α (GADD45A), a signaling protein whose expression in skeletal muscle rises during aging and a wide range of illnesses and injuries. We found that GADD45A induced several cellular changes that are characteristic of skeletal muscle atrophy, including a reduction in skeletal muscle mitochondria and oxidative capacity, selective atrophy of glycolytic muscle fibers, and paradoxical expression of oxidative myosin heavy chains despite mitochondrial loss. These cellular changes were at least partly mediated by MAP kinase kinase kinase 4, a protein kinase that is directly activated by GADD45A. By inducing these changes, GADD45A decreased the mass of muscles that are enriched in glycolytic fibers, and it impaired strength, specific force, and endurance exercise capacity. Furthermore, as predicted by data from mouse models, we found that GADD45A expression in skeletal muscle was associated with muscle weakness in humans. Collectively, these findings identify GADD45A as a mediator of mitochondrial loss, atrophy, and weakness in mouse skeletal muscle and a potential target for muscle weakness in humans.

Decreased skeletal muscle strength and mitochondrial dysfunction are characteristic of diabetes. The actions of insulin and IGF-1 through the insulin receptor (IR) and IGF-1 receptor (IGF1R) maintain muscle mass via suppression of forkhead box O (FoxO) transcription factors, but whether FoxO activation coordinates atrophy in concert with mitochondrial dysfunction is unknown. We show that mitochondrial respiration and complex I activity were decreased in streptozotocin (STZ) diabetic muscle, but these defects were reversed in muscle-specific FoxO1, -3, and -4 triple-KO (M-FoxO TKO) mice rendered diabetic with STZ. In the absence of systemic glucose or lipid abnormalities, muscle-specific IR KO (M-IR-/-) or combined IR/IGF1R KO (MIGIRKO) impaired mitochondrial respiration, decreased ATP production, and increased ROS. These mitochondrial abnormalities were not present in muscle-specific IR, IGF1R, and FoxO1, -3, and -4 quintuple-KO mice (M-QKO). Acute tamoxifen-inducible deletion of IR and IGF1R also decreased muscle pyruvate respiration, complex I activity, and supercomplex assembly. Although autophagy was increased when IR and IGF1R were deleted in muscle, mitophagy was not increased. Mechanistically, RNA-Seq revealed that complex I core subunits were decreased in STZ-diabetic and MIGIRKO muscle, and these changes were not present with FoxO KO in STZ-FoxO TKO and M-QKO mice. Thus, insulin-deficient diabetes or loss of insulin/IGF-1 action in muscle decreases complex I-driven mitochondrial respiration and supercomplex assembly in part by FoxO-mediated repression of complex I subunit expression.