Protein Abundance of NOR‐1 is Diminished During Aging and Skeletal Muscle Disuse

Abstract

INTRODUCTION Skeletal muscle mass and function are crucial for metabolic health and quality of life. Mitochondrial defects and loss of proteostasis driven by inactivity and aging predispose skeletal muscle to apoptosis and greater degradative signaling. Nuclear orphan receptor NOR-1 has been shown to be responsive to exercise and promotes an oxidative phenotype. NOR-1 is also an important regulator of glucose uptake, fatty acid oxidation, and mitochondrial biogenesis. A gap in our knowledge is that we do not know if NOR-1 is altered by aging or muscle disuse. PURPOSE Exercise is beneficial for skeletal muscle and overall health. However, exercise participation is low among the elderly, which represent a clinically relevant population prone to inactivity and bed rest. Disuse due to prolonged immobility can result in significant atrophy and metabolic impairments to the immobilized limb. Identifying genes that are dysregulated during aging or disuse is crucial for discovering therapeutic targets that might be modulated to delay or mitigate detriments to skeletal muscle mass and function. Although NOR-1 is responsive to exercise and is associated with improved metabolic and mitochondrial function, the role of NOR-1 in disuse and aging-related muscle atrophy has yet to be investigated. Therefore, the purpose of this study was to determine if aging and limb immobilization impact skeletal muscle protein content of NOR-1. We hypothesized that both disuse and aging reduce skeletal muscle protein abundance of NOR-1 in mice. METHODS Aged (33 months) and adult (6-10 months) C57/BL6 mice were used in this study. To investigate the effect of age, the tibialis anterior (TA) of aged and adult control mice were harvested and analyzed for skeletal muscle protein content of NOR-1. In a separate set of experiments, adult mice were subjected to 14 days of unilateral cast immobilization to induce disuse associated atrophy in the hindlimb plantarflexors. After immobilization, the gastrocnemius and soleus muscles of both the immobilized and contralateral limb were harvested, weighed, and analyzed for NOR-1 protein content. Skeletal muscle abundance of NOR-1 protein was measured by western blot. Paired t-test were used to determine differences between immobilized and contralateral limbs. To compare aged and adult animals, data were analyzed via unpaired t-test. RESULTS Protein abundance of NOR-1 was reduced (72%; p<0.05) in aged mice when compared to adult control mice. Additionally, hindlimb immobilization in adult mice resulted in marked (35% and 18%; p<0.05) decrease in the wet weight of soleus and gastrocnemius muscles respectively and reduced NOR-1 protein abundance (51% and 42%; p<0.05) in soleus and gastrocnemius muscles respectively, as compared to the contralateral control muscles. Taken together, these data suggest muscle protein content of NOR-1 is reduced during disuse and aging.