Metformin and leucine increase satellite cells and collagen remodeling during disuse and recovery in aged muscle.

Loss of muscle mass and strength after disuse followed by impaired muscle recovery commonly occurs with aging. Metformin (MET) and leucine (LEU) individually have shown positive effects in skeletal muscle during atrophy conditions but have not been evaluated in combination nor tested as a remedy to enhance muscle recovery following disuse atrophy in aging. The purpose of this study was to determine if a dual treatment of metformin and leucine (MET + LEU) would prevent disuse-induced atrophy and/or promote muscle recovery in aged mice and if these muscle responses correspond to changes in satellite cells and collagen remodeling. Aged mice (22-24 months) underwent 14 days of hindlimb unloading (HU) followed by 7 or 14 days of reloading (7 or 14 days RL). MET, LEU, or MET + LEU was administered via drinking water and were compared to Vehicle (standard drinking water) and ambulatory baseline. We observed that during HU, MET + LEU resolved whole body grip strength and soleus muscle specific force decrements caused by HU. Gastrocnemius satellite cell abundance was increased with MET + LEU treatment but did not alter muscle size during disuse or recovery conditions. Moreover, MET + LEU treatment alleviated gastrocnemius collagen accumulation caused by HU and increased collagen turnover during 7 and 14 days RL driven by a decrease in collagen IV content. Transcriptional pathway analysis revealed that MET + LEU altered muscle hallmark pathways related to inflammation and myogenesis during HU. Together, the dual treatment of MET and LEU was able to increase muscle function, satellite cell content, and reduce collagen accumulation, thus improving muscle quality during disuse and recovery in aging.