OBJECTIVES:
Musculoskeletal loss and the associated functional impairment affect broadly, including the elderly people, patients with chronic diseases such as cancer, and astronauts. Microgravity in space breaks tissue homeostasis in skeletal muscle by activating proteolysis and inflammatory pathways, leading to muscle atrophy. SIRT1, a NAD+-dependent protein deacetylase, is a critical gene regulating metabolism and tissue homeostasis in skeletal muscle. Notably, activating SIRT1 inhibits protein degradation in skeletal muscle by counteracting the ubiquitin proteasome pathway. In addition, our recent results showed that activating SIRT1 by nicotinamide mononucleotide (NMN), an NAD+ precursor, reverses functional decline in skeletal muscle of aged mice by mimicking exercise. All of this evidence suggests that maintaining high NAD+ levels in muscle tissues is a practical and safe intervention strategy for preventing muscle atrophy. The goal of this proposal is to test if boosting NAD+ mitigates unloading-induced musculoskeletal loss. Specifically, we will investigate the following objectives.
Objective 1: Determine the effect of NMN on mitigating unloading-induced musculoskeletal loss. We will use hindimb suspension (HS) in mice to simulate microgravity-induced muscle unloading. Our hypothesis is NMN administration during unloading mitigates muscle atrophy, bone loss, and functional impairment. We will also investigate if NMN alleviates slow-to-fast fiber type shift caused by muscle unloading, which significantly reduces fatigue resistance of the slow-twitch muscles.
Objective 2: Determine if NMN improves the effectiveness of exercise during unloading. As an exercise mimetics, NAD+ promotes the beneficial effects of exercise by activating SIRT1. We propose that raising NAD+ levels during exercise confers additive benefits than does exercise alone. We will test if SIRT1 overexpression or NMN administration augment the effectiveness of exercise and further mitigate
musculoskeletal loss.
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APPROACH:
Objective 1. Determine the effect of NMN on mitigating unloading-induced musculoskeletal loss. Muscle homeostasis is maintained by a balance between protein synthesis and turnover. During spaceflight, muscle unloading breaks this balance and induces a dramatic increase of protein degradation, leading to muscle atrophy (Baldwin et al., 2013). Abundant evidence has established that SIRT1 is a key regulator of metabolism and proteolysis in skeletal muscle (Lee and Goldberg, 2013; Gomes et al., 2013; Price et al., 2012). Notably, our lab has shown that activating SIRT1 by NMN significantly blocks induction of the ubiquitin proteasome pathway in atrophying muscles of aged mice, and rescues the mitochondrial function to a youthful level (Gomes et al., 2013). Besides muscle atrophy, unloaded muscle in microgravity undergoes a slow-to-fast fiber type shift, which severely impairs the fatigue resistance of slow-twitch muscles (Trappe et al., 2009). Activating SIRT1 by NMN is expected to antagonize this process by stimulating PGC-1a, a driver of slow-twitch muscle fiber formation (Lin et al., 2002). In this objective, we will test the hypothesis that NMN will alleviate muscle loss, slow-to-fast myofiber transition, and functional decay during unloading.
Objective 2. Determine if NMN improves the effectiveness of exercise during unloading. Physical exercise is still the most effective countermeasure to mitigate musculoskeletal loss in space (Petersen et al., 2016). Functioning as an exercise mimetics, NMN increases NAD+ levels and activates SIRT1 in skeletal muscle (Fan and Evans, 2017). SIRT1 is not only a critical gene regulating anabolic and catabolic pathways in the muscle (see Objective 1), but involved in regulating mitochondria function and biogenesis, a dictator of energy production and overall condition of the muscle (Gomes et al., 2013; Price et al., 2012). Moreover, our accepted work in Cell showed that SIRT1 is required for exercise-induced neovascularization in skeletal muscle (Das et al., Cell. Accepted). We found that NMN intake combined with training could further increase the capillary density in quadriceps in mice, suggesting a synergistic effect between raising NAD+ levels and exercise. In this objective, we will first test if SIRT1 activity is a factor that determines the exercise outcomes. Based on our preliminary results, we will further determine if NMN improves the effectiveness of exercise countermeasures on preserving muscle mass, strength, and endurance during unloading.
RESULTS:
This experiment is currently in progress. Results will be available at the conclusion of the study.
