Numerous lines of evidence have suggested that energy balance in animals and humans is tightly controlled. With the identification of leptin and its receptors by our laboratory, two of the molecular components of a system that maintains constant weight have been identified. Leptin is a hormone secreted by the 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 animals 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. Recently, leptin was approved by the US Food and Drug Administration (FDA) for the treatment of severe lipodystrophy, a condition with extreme insulin resistance and diabetes resulting from a loss of fat tissue.
Our current research focuses on a series of questions pertinent to the regulation of body weight: How does the fat cell regulate how much leptin is made—i.e., how does the fat cell know how fat it is? How does a single molecule (leptin) change feeding, a complex behavior? How do brain pathways that are modulated by leptin in turn regulate peripheral metabolism and insulin action? Do variations in the genes that compose the physiologic circuit of which leptin is a component explain differences in body weight?
Neural Circuitry Controlling Feeding
The recent identification of the hypothalamic cells that express the leptin receptor is enabling us 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 hedonic value of nutrient (i.e., the pleasure associated with food). This is important because it shows that the pleasure we derive from eating is not fixed but rather reflects the status of metabolic signals such as leptin. We have also identified a specific neural population in the hypothalamus that expresses a bioactive peptide known as MCH (melanin-concentrating hormone) that plays a key role in sensing the reward value of food. Our ongoing studies seek to understand how leptin modulates the activity of these neurons. We have also developed several new methods for identifying additional neural populations that regulate feeding. In the first, a novel gene-profiling technology can be used to define neural populations whose activities are regulated by a stimulus. In the second, gene-profiling technology is used to define specific neural populations based on their anatomic connections. Finally, we have developed a new method for probing neural function by using magnets or radio waves to remotely modulate the activity of specific neural populations in vivo.
Regulation of Leptin Production
We are also studying the molecular mechanisms responsible for changes of leptin gene expression associated with changes in fat mass. The amount of leptin that is expressed from fat is strongly regulated, with a 100-fold or more level of expression from ob/ob adipose tissue than from the adipose tissue of a lean or fasted animal. This suggests that the fat cell knows how much fat it has and adjusts leptin expression accordingly. The underlying mechanism responsible for this regulation is unknown. To address this question, we are using transgenic mice to identify DNA regulatory elements that change expression of a reporter gene controlled by the leptin gene in proportion to changes in adipose tissue mass. We have thus modified a series of leptin bacterial artificial chromosome clones so that the leptin DNA regulatory elements direct the expression of luciferase. This has enabled us to identify DNA regulatory sequences that control leptin gene expression. Protein factors binding to these sequences have also been identified, and studies to elucidate how the activity of these factors changes as fat is gained or lost are under way. We hypothesize that these studies will lead to the identification of a novel lipid-sensing signaling pathway in adipocytes and possibly other cell types.
Molecular Basis of Leptin's Metabolic Effects
Leptin, which has potent metabolic effects to improve insulin action and reduce the lipid content of peripheral tissues, is now an FDA-approved drug for the treatment of lipodystrophy, a severe form of human diabetes. We are studying the mechanism responsible for leptin’s antidiabetic function in this and other forms of diabetes. Current data from our laboratory and others suggest that leptin interferes with both the production and action of glucagon, which is another hormone that acts to increase blood glucose by opposing the effects of insulin. The cellular mechanisms responsible for this are under investigation.
Genetic Studies in Humans
Advances in genetics make it possible to use high-throughput DNA sequencing to identify genes that contribute to human disease. To implement this approach, we have been conducting genetic studies in collaboration with Tayfun Ozcelik (Bilkent University, Ankara, Turkey). Genetic studies can be facilitated by studying patients who are children of first-cousin (consanguineous) marriages, which are common in the Middle East. Ozcelik and his colleagues are collecting consanguineous pedigrees that include patients who are morbidly obese or extremely lean or have polycystic ovary syndrome (PCOS), which is associated with extreme resistance to insulin. More than 50 such families have been collected thus far, and we plan to collect several hundred more. Whole-genome sequencing is now under way, with the expectation that analyses of the DNA sequences from patients who are lean or obese will reveal DNA mutations that contribute to differences in weight or, in other families, lead to PCOS.
Grants from the National Institutes of Health and the JPB Foundation provided partial support for these projects.
As of May 16, 2014