Effect of aging, endurance training, and denervation on innate immune signaling in skeletal muscle

Skeletal muscle function relies on mitochondria for energy and for mediating its unique adaptive plasticity. The NLRP3 inflammasome complex is an innate immune mechanism that responds to mitochondrial damage-associated molecular patterns (DAMPs); however, its activity relative to mitochondrial dysfunction in muscle requires exploration. The purpose of this study was to characterize immune signaling and mitochondrial function in muscle during aging, endurance training, and disuse induced by denervation. Denervation led to decreases in muscle mass, mitochondrial content, and impaired respiration. Protein analyses revealed increases in NF-κB p65 and downstream inflammatory markers, including NLRP3, caspase-1, GSDMD-N, STING, and IL-1β, along with pro-apoptotic BAX and AIF. When assessing potential DAMPS, denervation led to increased ROS production but no changes in cytosolic mtDNA levels, relative to total mtDNA. Since we hypothesized that inflammasome activation would be increased with age, we studied young (6-8 mo) and aged (21-22 mo) mice that remained sedentary or underwent a 6-wk voluntary running protocol. Aging resulted in marked increases in the expression of multiple pro-inflammatory and pro-apoptotic proteins. Remarkably, training uniformly attenuated age-related increases in BAX, NLRP3, caspase-1, STING, and gasdermin-D (GSDMD) protein expression and tended to reduce the elevated level of cytosolic mtDNA evident in aged muscle. Training adaptations were also evident in the aged animals by the preservation of muscle mass and improvements in oxygen consumption and endurance performance, and were achieved despite a lower training distance than in young animals. Our results strongly implicate endurance training as a promising therapeutic for combating disuse and age-related inflammation in skeletal muscle.<b>NEW & NOTEWORTHY</b> This study aims to further elucidate the role between skeletal muscle activity, mitochondria, and innate immune signaling, specifically looking at the NLRP3 inflammasome complex. Our results demonstrate that disuse conditions such as aging and denervation were associated with a pro-inflammatory phenotype and impairments in mitochondrial functioning, whereas chronic endurance training demonstrated anti-inflammatory effects. Notably, this work highlights the benefits of exercise in attenuating age-related inflammation in aged skeletal muscle.