SAN DIEGO - Scientists working with genetically engineered mice at the Salk Institute discovered a molecular "switch" that enhances the animals' stamina and appears to burn fat, the group reported today.

The discovery of the switch, a receptor called PPAR-delta, could lead to treatments in people for obesity and related disorders such as heart disease and diabetes, according to the Salk Institute study.

The study, led by professor Ronald Evans and postdoctoral fellow Yong-Xu Wang, will be published in the September issue of the Public Library of Science Biology journal.

Researchers at the institute discovered that activating PPAR-delta increases the rate at which the body burns fat.

The team produced a genetically engineered mouse with the activated form of PPAR-delta in its skeletal muscles. The result was an increase in muscle cells and a mouse capable of running up to twice the distance of a normal, untrained mouse from the same litter, according to the study.

"By expressing genes for an activated form of the receptor PPAR-delta, we created a mouse that could, compared to normal mice, run marathons," said Evans, who is also an investigator at Howard Hughes Medical Institute.

"These muscles also provided resistance to obesity, despite the level of exercise," said Evans. "By manipulating this receptor, it is possible to design treatments that change our muscle makeup and help resist obesity and associated metabolic disorders."

To test the results, the researchers treated normal mice with an experimental drug called GW501516, which activates PPAR-delta. Those mice also produced more efficient muscle fibers and gained less weight when given a high-fat diet and no exercise.

The drug is in its early stages of being tested on people for its effects on obesity and other disorders related to fat metabolism.