Promoting Skeletal Regeneration in Aged Mice Summary In this study we will investigate the mechanisms behind failed bone and soft tissue regeneration in the aged digit amputation mouse model. While the digit regeneration model has gained considerable traction in recent years and resulted in a substantial amount of knowledge regarding mechanisms underlying regeneration, this is the first study that will use this model to explore regeneration the context of aging. We show that regeneration fails in this model in ways that are similar to compromised bone turnover in old age, including exacerbated bone degradation activity and the attenuated ability to mount a successful regenerative bone response. This age-related response is similar to the one seen when we applying increased oxygen tension, disrupting the native oxygen microenvironment in the regenerating digit, which requires an oscillation between both hypoxic and normoxic oxygen environments for successful regeneration. While it is clear that the oxygen microenvironment of the digit is dynamic and critical to successful regeneration, it is not known how oxygen cues the regenerative process and whether or not this is directly linked to changes in cell metabolism or metabolic switching, as is seen in certain cancers. The studies outlined in this project will explore the mechanisms through which regeneration fails during aging by specifically investigating the role of oxygen signals and cellular metabolism. We will test the hypothesis that regeneration fails in aging as a direct result of the inability of key cell populations to successfully execute metabolic switching between mitochondrial respiration and glycolytic activity and to respond to changes in the oxygen microenvironment. To test this hypothesis we will evaluate regeneration in both aged outbred mice (Aim 1) and in a transgenic mouse model of aging with metabolic syndrome (Aim 2), and test the ability of these two strains of mice to respond effectively to changes in oxygen levels (Aim 3). This project will provide valuable data in both the regeneration and aging fields and will yield new mechanistic insights that will help guide future therapeutic intervention.