CRESPI, Erica J*; DENVER, Robert J; Univ. of Michigan, Ann Arbor: Conserved and novel functions of leptin in the South African clawed frog (Xenopus laevis)

Leptin, the protein product of the obese (ob) gene, is a type-I cytokine hormone secreted by fat that is integral to food intake regulation and influences almost every physiological system in juvenile and adult mammals. As a first step in studying the physiological roles of leptin in amphibians, we cloned the ob gene and a cDNA fragment of the leptin receptor gene in Xenopus laevis. The distribution of mRNA expression of leptin and its receptor in juvenile X. laevis suggests that leptin may have autocrine, paracrine, and hemocrine functions in amphibians. We also produced and purified recombinant X. laevis leptin (rxLeptin) in E. coli, and showed that intracerebroventricular injection of rxLeptin inhibited food intake in late-staged tadpoles and frogs as shown in mammals. rxLeptin treatment did not affect food intake in early prometamorphic tadpoles; however, repeated rxleptin i.p. injections induced growth and development of the hind limb. This effect was specific to the hind limb, as tail length, tailfin height, body length, and body weight were not affected by rxleptin treatment. We found that leptin receptor mRNA, but not leptin mRNA, is expressed in the hind limb of early prometamorphic tadpoles. Furthermore, leptin treatment increased [3H]-thymidine uptake by cultured hind limbs, suggesting that rxleptin can directly stimulate cell proliferation in this tissue. These findings suggest that the function of leptin in the regulation of appetite and energy balance evolved prior to the divergence of amniotes in the vertebrate lineage. In addition, we show for the first time that leptin influences limb growth and differentiation during early development (supported by NSF grant IBN 0235401 to R.J.D.)