Nutrient scarcity underlies transient loss of regenerative potential in Xenopus tropicalis Jeet H. Patel, Madison C. Williams, Andrea E. Wills Department of Biochemistry, University of Washington, Seattle WA Program in Molecular and Cellular Biology, University of Washington School of Medicine, Seattle WA Institute for Stem Cell and Regenerative Medicine, University of Washington, Seattle WA Regenerative potential varies greatly between vertebrate species. Mammals have relatively limited capacity for restoring organs and limbs, while animals such as axolotls and zebrafish will replace lost or damaged structures will fully functional, properly patterned substitutes. The generation of entirely new tissue in these animals is a metabolically expensive process which necessitates a complex injury response followed by substantial biosynthesis and cell division. We used the model Xenopus tropicalis to investigate how nutrient availability factors into regenerative potential. Previous work has shown that the closely related cousin species, X. laevis, has a brief window of development in which regenerative potential for the tail sharply declines from Nieuwkoop-Faber Stages 45-47. We find that in X. tropicalis, these developmental stages correlate with a gradual reduction in tail regeneration, with limited regrowth at stage 47. The drop-off in regenerative potential coincides with the transition from maternal yolk to independent feeding, and correlates strongly with a rapid depletion of the maternal yolk protein, vitellogenin. We hypothesized that as available nutrient stores in the yolk decline, tadpoles are unable to expend limited metabolic fuel to sustain growth. To test this, we compared tadpoles fed for different periods of time prior to stage 47 to see if independent feeding could restore regeneration. Indeed, feeding tadpoles for 24 hours prior to injury allowed them to robustly regenerate, but addition of food at the time of injury does not. We hypothesized that the ability to detect nutritive surplus might be required to initiate regeneration and tested if mTOR, a major nutrient sensing pathway, activity was required for regeneration. We found that treatment of fully regenerative animals prior to the refractory period with mTOR antagonists significantly reduced regeneration length, suggesting that nutrient sensing may play a role in permitting growth following injury. These findings highlight that nutrient availability is a critical part of the regenerative process and underscore the need for understanding how metabolic process are interacting with known genetic and physiological determinants of regeneration.