Many phylogenetically diverse taxa can regenerate injured or excised body parts, including both invertebrate and vertebrate species, but the ability to regenerate structures varies with environmental conditions, ranging from natural abiotic factors to anthropogenic factors. Very little is known about the factors that transduce these environmental cues to the cellular processes at the site of regeneration, although there is evidence that neuroendocrine systems are involved. Here, we tested the hypothesis that leptin, a peptide hormone related to nutritional status, regulates limb regeneration through neuroendocrine pathways in the South African clawed frog, Xenopus laevis. The ability to regenerate limbs is primarily limited to amphibians among vertebrates, with evidence indicating the initial stages of regeneration are the most important for the success of limb regeneration. Previous studies showed that intraperitoneal (ip) and intracerebroventricular (icv) leptin injections enhance limb development in Nieuwkoop-Faber v (NF) stage 52-53 tadpoles, raising the possibility that leptin also enhances limb regeneration in tadpoles. In this study, we found that central administration of leptin into NF stage 53 tadpoles at the time of limb amputation resulted in a significant increase in the area regenerated. We also showed that leptin receptors were activated in the tadpole ventral hypothalamus and the pituitary gland 6 hours after ip leptin injections, as shown by increased immunoreactivity of phosphorylated signal transducer and activator of transcription 3 (p-STAT-3). This suggests that leptin could stimulate hypothalamic releasing factors or pituitary cells to secrete factors that increase regeneration. Although we did not detect leptin-induced p-STAT-3 immunoreactivity in growth hormone secreting cells, or somatotrope cells, in the pituitary, we found p-STAT-3 staining in the anterior pituitary. Given that lactotropes are located in the anterior pituitary, future studies will test whether leptin stimulates the secretion of prolactin, which advances regeneration in other amphibians and lizards. For the first time it was demonstrated that leptin has enhancing effects on limb regeneration through neuroendocrine pathways in X. laevis tadpoles. Identifying the enhancers of regeneration in amphibians could provide novel factors that could be used to regenerate mammalian tissues in vitro or in vivo
