OBJECTIVE: ASTRONAUTS LOSE 1-1.5% OF THEIR BONE MASS PER MONTH DURING LOW EARTH ORBIT MISSIONS. SPACEFLIGHT PROFILES OF 2 YEARS OR LONGER AS PROJECTED FOR MARS MISSIONS WILL SIGNIFICANTLY INCREASE BONE FRAGILITY POTENTIALLY COMPROMISING THE ABILITY OF ASTRONAUTS TO PERFORM THEIR MISSION TASKS IN TRANSIT AND ON MARS. TO FACILITATE HUMAN EXPLORATION OF MARS IT IS THEREFORE ESSENTIAL TO UNDERSTAND THE UNDERLYING CAUSES OF MICROGRAVITY-INDUCED BONE LOSS AND DEVELOP EFFECTIVE COUNTERMEASURES TO SUSTAIN BONE STRUCTURE AND MECHANICAL INTEGRITY UNDER MICROGRAVITY CONDITIONS. OUR LONG-TERM GOAL IS TO PROTECT THE SKELETAL HEALTH OF ASTRONAUTS IN SPACE ENVIRONMENTS SO THEY CAN PERFORM THEIR MISSION OPERATIONS AND RETAIN THEIR QUALITY OF LIFE UPON RETURN TO EARTH. THE OBJECTIVE OF THE CURRENT PROPOSAL IS TO DETERMINE THE EFFECTS OF MICROGRAVITY AND PARTIAL MICROGRAVITY ON BONE REMODELING AS WELL AS EVALUATE HYPERGRAVITY AS A COUNTERMEASURE FOR MICROGRAVITY-INDUCED BONE LOSS. OUR CENTRAL HYPOTHESIS IS THAT SIMULATED MICROGRAVITY (SPACE 0G) AND PARTIAL MICROGRAVITY (MARS 1/3G MOON 1/6G) WILL IMPAIR BONE REMODELING AND THAT HYPERGRAVITY WILL MITIGATE MICROGRAVITY-INDUCED BONE LOSS. THE RATIONALE FOR THIS PROPOSAL IS THE FACT THAT MICROGRAVITY AND PARTIAL MICROGRAVITY CAUSE BONE LOSS BY REDUCING MECHANICAL STRESSES ON BONE. THEREFORE THE APPLICATION OF PERIODIC HYPERGRAVITY WILL RESTORE NORMAL BONE STRESSES THEREBY RESCUING PHYSIOLOGICAL REMODELING. FURTHER STUDY WILL ALSO LOOK AT RECOVERY RATE IN DIFFERENT MICROGRAVITY CONDITIONS WITH AND WITHOUT PERIODIC HYPERGRAVITY EXPOSURE. SPECIFIC AIMS AND METHODS: THE CENTRAL HYPOTHESIS WILL BE TESTED BY COMPLETING THE FOLLOWING TWO SPECIFIC AIMS. 1-DETERMINE THE EFFECTS OF MICROGRAVITY AND PARTIAL MICROGRAVITY ON BONE LOSS AND LONG-TERM RECOVERY USING RAT SUSPENSION MODELS. OUR WORKING HYPOTHESIS IS THAT MICROGRAVITY (0G) AND PARTIAL MICROGRAVITY (1/3G AND 1/6G) WILL ALTER BONE REMODELING LEADING TO A LONG-TERM BONE LOSS. MICROCT AND HISTOMORPHOMETRIC ANALYSIS OF BONE FORMATION AND RESORPTION WILL BE CONDUCTED TO DETERMINE MICROSTRUCTURAL CHANGES. MECHANICAL TESTING WILL BE USED TO ASSESS STRUCTURAL INTEGRITY. THE GENOMIC EFFECTS OF MICROGRAVITY WILL BE EVALUATED USING RNASEQ TO IDENTIFY DIFFERENTIALLY EXPRESSED SIGNALING PATHWAYS. LONGITUDINAL ANALYSIS OF SERUM LEVELS OF BONE MARKERS WILL BE CONDUCTED USING ELISA ANALYSIS. 2-ASSESS THE EFFICACY OF PERIODIC HYPERGRAVITY DURING SIMULATED MICROGRAVITY AND PARTIAL MICROGRAVITY AS COUNTERMEASURES TO MITIGATE MICROGRAVITY-INDUCED SHORT TERM AND LONG TERM EFFECTS ON BONE INTEGRITY. OUR WORKING HYPOTHESIS IS THAT HYPERGRAVITY WILL PROVIDE ESSENTIAL MECHANICAL STIMULATION TO RESTORE MICROGRAVITY AND PARTIAL MICROGRAVITY-INDUCED IMPAIRMENT OF BONE REMODELING. THE EFFICACY OF PERIODIC HYPERGRAVITY WILL BE DETERMINED USING BONE MICROSTRUCTURAL ANALYSES (MICRO CT BONE FORMATION AND RESORPTION HISTOMORPHOMETRY) MECHANICAL ANALYSIS (THREE-POINT TESTING) GENOMIC (RNA SEQ) AND SERUM BIOMARKER ANALYSIS (ELISA). SIGNIFICANCE AND OUTCOMES: BONE REMODELING IS A COMPLEX PROCESS THAT IS TIGHTLY REGULATED BY OSTEOBLAST-OSTEOCLAST COUPLING AND COMMUNICATION. MICROGRAVITY IMPAIRS THIS PROCESS BY INCREASING OSTEOCLAST ASSOCIATED RESORPTION AND DECREASING OSTEOBLASTIC BONE FORMATION. IN ORDER TO CONTINUE TO EXPAND OUR MANNED SPACEFLIGHT PROGRAM IT IS ESSENTIAL TO DETERMINE THE EFFECTS OF MICROGRAVITY ON BONE REMODELING AND DEVELOP TARGETED COUNTERMEASURES TO SUSTAIN BONE HEALTH IN MICROGRAVITY AND PARTIAL MICROGRAVITY. AT THE COMPLETION OF THE STUDY WE EXPECT TO CHARACTERIZE THE EFFECTS OF MICROGRAVITY AND PARTIAL MICROGRAVITY ON BONE STRUCTURAL AND MECHANICAL INTEGRITY IDENTIFY GENOMIC EFFECTS OF MICROGRAVITY EXPOSURE AND ESTABLISH THE EFFICACY OF PERIODIC HYPERGRAVITY TO RESTORE PHYSIOLOGICAL BONE REMODELING. THE OUTCOMES OF THIS STUDY WILL FILL NASAS RECOGNIZED KNOWLEDGE GAS (OSTEO 4 AND 7) AND RESEARCH EMPHASIS 3 AND 4 (APPENDIX G).