Meeting Abstract
Leptin is a pro-inflammatory adipokine hormone, modulating both adaptive and innate immune responses in mammals, but its role in the immune system of other vertebrates is poorly understood. Previously, we showed that leptin has pro-inflammatory actions and stimulates splenocyte proliferation in juvenile X. laevis. Leptin treatment also reduces mortality in tadpoles with bacterial infections. To determine whether leptin can have direct actions on immune organs in X. laevis, we used in situ hybridization to detect long form leptin receptor (LRb) mRNA in the thymus, spleen, liver, gut, and skin of tadpoles and juveniles. In tadpoles (NF St. 56), LRb is expressed in thymus and skin melanocytes, but absent in the spleen, potentially because in the spleen are specialized melano-macrophages. Melanocytes can act as signaling centers regulating apoptosis during metamorphosis. Leptin activation of melanocytes could indirectly affect cell number in these organs through both directly and by stimulating release of αMSH. In juveniles, LRb was expressed in spleen cortex and may have a greater role in adaptive immunity at later stages. We showed using antibody staining that leptin is most concentrated in the gut and liver, indicating these tissues may be secreting it. The spleen and thymus had lower expression levels in solitary cells spread throughout the organ. In the thymus, leptin was also found in the individual melanosomes. This research suggests that leptin directly regulates immunity through activation of LRb in both tadpole and juvenile amphibians, although it may have a greater role in adaptive immunity after metamorphosis. We also show leptin may have paracrine or autocrine actions, as both leptin and LRb are expressed in larval melanocytes, a novel role for leptin in the regulation of melanocytes.
