PROJECT SUMMARY Duchenne Muscular Dystrophy (DMD) is the most common muscular dystrophy seen in children, which affects both muscle and bone. To date, the mechanisms that contribute to the poor skeletal health in DMD are not fully understood and no targeted therapies exist to slow or halt the progression of osteoporosis and related fractures. We have identified a novel bone-regulating myokine, FGF-21, which is typically not expressed in skeletal muscle under physiological conditions, is dramatically increased in dystrophic skeletal muscles in DMD mouse models. However, the mechanisms which drive FGF21 expression in dystrophic muscle and the pathological role of FGF21 on bone metabolism in DMD are not clear, representing a significant gap in knowledge. The purpose of this proposal is to identify the cellular source of elevated FGF21, determine the molecular mechanism(s) that drive FGF21 expression, and to characterize the pathological effects of muscle-derived FGF21 on bone in DMD. The cellular and molecular mechanisms of upregulated FGF21 expression will be determined in skeletal muscle of dystrophic mouse models. The pathological role of muscle-derived FGF21 on skeletal muscle, bone and whole-body metabolism will be determined by skeletal muscle conditional FGF21 knock out animals and in in vitro cell models. Completion of these studies will not only provide understanding of the pathogenesis of DMD skeletal abnormalities but also elucidate a novel muscle/bone crosstalk signaling pathway mediated via myokine FGF21. The proposed studies will provide the groundwork for developing potential therapies targeting FGF21 signaling to manage poor bone health in DMD patients.