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 THESE 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 EXERCISE ALONE. WE WILL TEST IF SIRT1 OVEREXPRESSION OR NMN ADMINISTRATION AUGMENT THE EFFECTIVENESS OF EXERCISE AND FURTHER MITIGATES MUSCULOSKELETAL LOSS. IT IS NOT CLEAR WHY THE CURRENT EXTENSIVE EXERCISE PROTOCOLS ARE NOT SUFFICIENT TO FULLY PREVENT MUSCLE ATROPHY IN SPACE. THIS PROPOSAL WILL FURTHER ELUCIDATE THE ROLE OF SIRT1 IN THE MAINTENANCE OF SKELETAL MUSCLE AND BONE AND TEST A VERY PROMISING STRATEGY TO RESIST MUSCLE ATROPHY DURING UNLOADING. IT WILL BENEFIT HUMAN SPACE EXPLORATION AND THE HUMANS ON EARTH THAT SUFFER FROM MUSCLE ATROPHY.