The proposed research is centered on understanding the hormone leptin's effects on bone. Leptin is a circulating protein hormone primarily known for its role in appetite control and fat deposition in mammals. Leptin also has many other effects, including influencing sexual maturity, the immune system, and bone metabolism. Even after hundreds of studies in mammals, there is not a consensus as to whether leptin's effects on bone increase or decrease bone volume. The comparative method, wherein organisms are compared with humans to understand similarities and differences among them, has paid great health dividends throughout the history of medicine (e.g. neuroscience using the squid axon, excretion using the shark rectal gland, and genetics using fruit flies). This proposal proposes to apply the comparative method by using zebrafish to help resolve the conflicting results of leptin bone studies in mice. We propose to use established knockout mutants of leptin and other proteins in its signaling pathway (mmp9 and endospanin) in zebrafish (Danio rerio). We will test the hypothesis that bone volume changes are a function of leptin sensitivity, and that this sensitivity can be manipulated with the protein endospanin. This proposal will address two Aims; Aim I manipulates leptin sensitivity in embryos, and Aim II manipulates leptin sensitivity in adults (documenting differences between males and females, and response under high fat diet). The proposal will be conducted at an institution and a laboratory that is dedicated to high quality research experiences for its undergraduates. Both PIs have a extensive experience as mentors and have trained many currently practicing health science researchers and physicians.