Friedman studies the molecular mechanisms that regulate food intake and body weight. Genetic studies in mice led him to identify leptin, a hormone made by fat tissue that plays a key role in controlling appetite and weight. His current work explores the mechanisms by which leptin mediates these functions, and seeks toidentify other key regulators of body weight.
Leptin is a hormone secreted by adipose (fat) tissue in proportion to its mass that in turn modulates food intake relative to energy expenditure. Increased fat mass increases leptin levels, which in turn reduces body weight; decreased fat mass leads to a decrease in leptin levels and an increase in body weight. By this mechanism, weight is maintained within a relatively narrow range. Defects in the leptin gene are associated with severe obesity in animal models and in humans.
Leptin acts on sets of neurons in brain centers that control energy balance to regulate appetite. Leptin also plays a general role in regulating many of the physiologic responses that are observed with changes in nutritional state, with clear effects on female reproduction, immune function, and the function of many other hormones, including insulin.
The recent identification of the hypothalamic cells that express the leptin receptor is enabling Friedman and his colleagues to delineate the precise neuronal effects of leptin and the mechanisms by which this single molecule can alter a complex behavior. Recent studies have revealed that leptin reduces food intake by decreasing the pleasure associated with food. Friedman’s lab has identified a specific neural population in the hypothalamus that expresses a bioactive peptide known as MCH, which plays a key role in sensing the reward value of food. His ongoing studies seek to understand how leptin modulates the activity of these neurons as well as to identify additional neural populations that regulate feeding.
The Friedman lab is also using transgenic mice to identify DNA regulatory elements that change expression of a reporter gene controlled by the leptin gene proportionately with changes in adipose tissue mass. They have modified a series of leptin bacterial artificial chromosome clones so that the leptin DNA regulatory elements direct the expression of luciferase, enabling them to identify DNA regulatory sequences that control leptin gene expression. The goal of these studies is to identify a novel lipid-sensing signaling pathway in adipocytes and possibly other cell types.
Leptin has potent metabolic effects to improve insulin action and reduce the retained lipid content of peripheral tissues and is now an FDA-approved drug for the treatment of severe lipodystrophy, a form of diabetes. The Friedman lab is studying the mechanism responsible for leptin’s antidiabetic function; current data suggest it interferes with both the production and action of glucagon, a hormone that acts to increase blood glucose by opposing the effects of insulin.
In collaboration with Tayfun Özçelik at Bilkent University in Ankara, Turkey, the Friedman lab is conducting studies of consanguineous families that include patients who are either morbidly obese, extremely lean, or have polycystic ovary disease (PCOS), which is associated with resistance to insulin. The team expects that analyses of the DNA sequences from these populations will reveal DNA mutations that contribute to differences in weight or that lead to PCOS.