Cloning and Characterization of Leptin in a Teleost Fish and its Role in Mediating Appetite and Growth

Mammalian and clinical studies show that leptin circulates as an endocrine reflection of fat deposition that relays information about energy reserves to the brain and periphery through its ubiquitously expressed receptor (OBR). The leptin system thereby modulates appetite and energy expenditure according to endogenous energy levels in what is referred to as a lipostatic model of energy homeostasis. Due to difficulty in sequencing leptin and OBR genes in lower vertebrates, however, limited information has been gathered about this system in ectotherms generally, including teleost fishes. Upon cloning a putative leptin gene in striped bass (Morone saxatilis), representing the first leptin identified in Perciformes, the largest and most diverse order of fish, we profiled its tissue distribution, measured gene expression during fed, fasted and refed conditions, assessed its effect on appetite and tested its regulatory influence on key elements of the endocrine growth axis. The OBR gene was also cloned and its mRNA measured under similar metabolic states in brain regions associated with appetite regulation. This research portrays a leptin system in Morone that adjusts energy intake and expenditure according to nutritional state. Leptin may thereby influence the shift away from feeding and toward growth during positive energy states, and therefore serve as an anabolic switch for metabolically expensive processes.;While genome duplication events have resulted in leptin paralogs in some fish lineages, the single Morone leptin appears to be orthologous to the medaka A-type. HSB leptin mRNA was exclusively expressed in the liver, which is an important energy storage organ in many fish, and is a prominent site of leptin gene expression in all fish studied to date. Hepatic leptin mRNA levels rose during feeding (anabolism) and decreased during fasting (catabolism) in hybrid striped bass (HSB; M. chyrsops x M. saxatilis). Leptin injection suppressed appetite. Together, its function as an anorexigen and its regulation according to metabolic state in HSB suggests that leptin may play a role in maintaining energy homeostasis in these fish.;The Morone OBR peptide sequence possesses all major features common to vertebrate OBRs. Central OBR transcript temporarily rose in the telencephalon during fasting, when leptin levels are expected to be low, and again during refeeding. The modulation of OBR gene expression during different metabolic states may be a means of adapting sensitivity to fluctuating plasma leptin concentrations in order to regulate feeding or leptin's other pleiotropic functions. Gene expression of neuropeptide Y (NPY), a potent appetite stimulant that is commonly suppressed by leptin, was also temporarily elevated in the telencephalon during fasting. The rise in NPY gene expression may therefore be part of a feeding mechanism by which periods of negative energy lead to an increase in appetite in order to replenish depleted energy reserves. The co-presence of NPY and OBR mRNAs in the telencephalon, along with their regulation by feeding status, suggests that this brain region plays a role in the central regulation of appetite.;Because hepatic leptin gene expression is indicative of anabolism, it was considered a candidate for regulating the endocrine growth axis. In this axis, circulating growth hormone (GH) stimulates production of insulin-like growth factors (IGF-I and IGF-II), mitogenic hormones that drive somatic growth, through GH receptor (GHR1 and GHR2) signaling in the liver. Growth occurs during positive energy states, whereas fasting leads to hepatic GH resistance and decreased IGF production. We show for the first time in vertebrates that leptin upregulates gene expression of both GHRs, as well as that of IGFs, suggesting a previously unrecognized function of leptin in coordinating growth with nutritional state, when energy and resources are available.