AIM 1: EXAMINE THE RESPONSE OF WILD-TYPE AND CX43-DEFICIENT (VIA CRISPR GENE EDITING) MURINE OSTEOCYTES (OCY454) TO SIMULATED MICROGRAVITY IN VITRO USING A HIGH ASPECT RATIO VESSEL (HARV). AIM 2: EXAMINE THE EFFECT OF SIMULATED MICROGRAVITY IN VIVO ON BONES FROM OSTEOCYTE-SPECIFIC CX43-DEFICIENT MICE. CX43 WILL BE SELECTIVELY DELETED FROM MICE USING A CRE RECOMBINASE APPROACH SIMILAR TO WHAT WE PREVIOUSLY USED TO DELETE CX43 FROM OSTEOBLASTS AND OSTEOCYTES. USING WELL-ESTABLISHED EXPERIMENTAL PARADIGMS LIKE THE HARV APPARATUS AND HLS GREATLY INCREASES THE PROBABILITY OF SUCCESSFULLY COMPLETING OUR AIMS AND ATTAINING IMPORTANT NOVEL FINDINGS. COLLECTIVELY OUR INFRASTRUCTURE AND EXPERTISE IN COMBINATION WITH MULTIPLE UNIQUE REAGENTS PLACE US IN AN UNPARALLELED POSITION TO RAPIDLY AND SIGNIFICANTLY ADVANCE KNOWLEDGE PERTAINING TO THE MECHANISMS UNDERLYING MICROGRAVITY-INDUCED BONE LOSS. THESE AIMS AND THE EXPERIMENTS DESIGNED THEREIN REPRESENT A SUBSTANTIVE DEPARTURE FROM OUR CURRENT UNDERSTANDING AND WILL REVEAL NOVEL TARGETS FOR COUNTERMEASURES.