IT IS WELL KNOW THAT EXTENDED SPACEFLIGHT CAUSES SIGNIFICANT SKELETAL MUSCLE ATROPHY IN ASTRONAUTS. HOWEVER THE MECHANISMS THAT DRIVE THIS PROCESS ARE NOT WELL UNDERSTOOD. PREVIOUS SPACE SHUTTLE EXPERIMENTS HAVE SHOWN THAT SPACEFLIGHT LEADS TO ATROPHY IN ISOLATED TISSUE ENGINEERED SKELETAL MUSCLE. HOWEVER WE DO NOT KNOW HOW MUSCLE TISSUES SENSE THE MICROGRAVITY ENVIRONMENT LEADING TO THE MUSCLE ATROPHY. SINCE THESE EARLIER FLIGHTS WITH TISSUE ENGINEERED SKELETAL MUSCLE WE HAVE DEVELOPED THE ABILITY TO MINIATURIZE THESE FUNCTIONAL MUSCLE UNITS AND TO ALSO MEASURE REAL-TIME FORCES. ADVANCES IN PROTEOMICS AND GENOMICS HAVE ALSO SIGNIFICANTLY ADVANCED OUR ABILITY TO BETTER UNDERSTAND MOLECULAR MECHANISMS OF MUSCLE PATHOLOGY. WE PROPOSE GROUND-BASED STUDIES TO FURTHER DEVELOP THESE MINIATURIZED TISSUE-ENGINEERED SKELETAL MUSCLE CONSTRUCTS AS AN IDEAL THREE-DIMENSIONAL FUNCTIONAL PHYSIOLOGICALLY RELEVANT IN VITRO MODEL FOR STUDY MICROGRAVITY-INDUCED SKELETAL MUSCLE ATROPHY. WE WILL VALIDATE A PREVIOUSLY DEVELOPED ATROPHY MODEL USING LARGER MUSCLE CONSTRUCTS BASED ON TENSION REDUCTION BY INVESTIGATING THE POTENTIAL USE OF A 3D CLINOSTAT AS A VALID GROUND-BASED MODEL FOR SIMULATING THE EFFECTS OF MICROGRAVITY. THESE DATA WILL PROVIDE THE BASIS FOR POTENTIAL FUTURE SPACEFLIGHT STUDIES THAT WILL ALLOW INVESTIGATORS TO MEASURE REAL-TIME FUNCTIONAL DATA IN MUSCLE AS THEY ATROPHY. IT WILL ALSO ENABLE THE EVALUATION OF VARIOUS ATROPHY COUNTERMEASURES DURING SPACEFLIGHT. OUR HYPOTHESIS IS THAT SIMULATED MICROGRAVITY INDUCES ATROPHY IN TISSUE ENGINEERED SKELETAL MUSCLE AND CAN BE ATTENUATED BY GROWTH FACTORS AND PHARMACEUTICALS. THE SUCCESSFUL COMPLETION OF THIS PROPOSAL WILL ESTABLISH THE DEVELOPMENT OF AN EFFICIENT HIGH THROUGHPUT GROUND-BASED IN VITRO MODEL SYSTEM FOR STUDYING THE EFFECTS OF MICROGRAVITY ON SKELETAL MUSCLE.