Leptin signaling in the central nervous system and the periphery.

The discovery of leptin in 1994 has led to astonishing advances in understanding the regulation of energy balance in rodents and humans. The demonstration of leptin receptors in hypothalamic regions known to play critical roles in regulating energy intake and body weight has produced considerable excitement in the field. Most attention has focused on the central actions of leptin. The receptor is present in several populations of neurons that express specific appetite-regulating neuropeptides for which both expression and release are regulated by leptin. Recent advances show that central leptin action is not limited to influencing energy balance. Leptin regulates a broad variety of processes and behaviors, such as blood pressure, neuroendocrine axes, bone mass, and immune function. The cloning of leptin receptors also led to parallel studies examining their signaling capacities in mammalian cell lines. The long-form receptor regulates multiple intracellular signaling cascades, including the classic janus activating kinase-signal transducer and activator of transcription (JAK-STAT) pathway, consistent with belonging to the cytokine-receptor superfamily and the phosphoinositol-3 kinase and adenosine monophosphate kinase pathways. Progress has been made in understanding the role of individual signaling pathways in vivo and the mechanisms by which specific neuropeptides are regulated. Regulation of the pro-opiomelanocortin (pomc) and the thyrotropin-releasing hormone (trh) genes by leptin is particularly well understood. Novel players in negative regulation of central leptin receptor signaling have been identified and open the possibility that these may be important in the development of leptin resistance and obesity. While initial focus was on the central effects of leptin, important actions have been discovered in peripheral tissues. These include roles of leptin to directly regulate immune cells, pancreatic beta cells, adipocytes, and muscle cells. Recent elucidation of a new signaling pathway in skeletal muscle affecting fatty acid metabolism has implications for regulation of insulin sensitivity and glucose metabolism. Recent progress in understanding central and peripheral leptin receptor signaling provides potential new targets for anti-obesity and anti-diabetes drug development.