The role of leptin in zebrafish (Danio rerio) : Novel insights into appetite regulation and reproduction

Abstract: The hormone leptin is a peripheral metabolic signal and an important regulator of energy balance. In mammals, leptin acts on the appetite centers in the hypothalamus, causing anorexigenic functions by inhibiting food intake. It is also considered as a link between the nutritional status and the endocrine reproductive axis. However, the actions of leptin in teleosts are not fully understood. This thesis investigated the possible role of leptin in the regulation of appetite and reproduction in teleosts, using a loss of function leptin receptor zebrafish strain (lepr sa12953).Under different feeding conditions (normal feeding, 7-day fasting, 2- and 6-hours post refeeding) the transcription of orexigenic and anorexigenic genes was influenced by leptin in the zebrafish brain. Leptin signaling inhibited the transcription of orexigenic genes, during short-term fasting and refeeding, and stimulated the transcription of anorexigenic genes under normal feeding in wild-types, indicating an anorexigenic role of leptin in appetite regulation in zebrafish. Moreover, a leptin-dependent gene regulatory network (GRN), involved in the behavioral and metabolic control of appetite was suggested in the brain, including the cart/crhb/gnrh2 genes and their respective co-expressed modules, mediated by the transcription factor sp3a.Furthermore, impaired leptin signaling resulted in severe reproductive deficiencies in female zebrafish. Folliculogenesis was not affected, but oocyte maturation and ovulation were disrupted in lepr mutant females, resulting in low number of ovulated eggs. Moreover, the transcripts of luteinizing hormone beta (lhb) in the pituitary were significantly lower in the mutant females. Analysis of candidate genes revealed differential expression of genes involved in steroidogenesis, oocyte maturation and ovulation in the ovaries of the lepr mutants. Transcriptomic analysis of isolated fully grown follicles linked the reproductive deficiencies to the suppression of essential metabolic pathways during oocyte maturation and ovulation in teleosts, such as estrogen regulation, ribosome biogenesis, mRNA translation and lipid metabolism.Overall, the results from the present thesis provided, for the first time in zebrafish, evidence that leptin is involved in appetite regulation, by mediating the transcription of appetite-regulating genes and a GRN in the brain, as well as that leptin consists a critical regulator of female reproduction, especially during oocyte maturation and ovulation.