Gene Initially Implicated in Obesity Appears to Play Role in Immune Responses and Production of Reactive Oxygen species

Duke University Medical Center
Wednesday, 29 November 2000

The group of researchers from Duke University Medical Center and France who found a gene in 1997 that appeared to keep mice from getting obese on a high-fat diet have now identified a new, and possibly more important, role for the gene - regulating potent immune responses.

The gene they discovered, called uncoupling protein 2 (UCP2), contains the blueprint for a heat-generating protein that appeared to play a role in regulating body temperature and burn excess calories as body heat before they can be stored as fat. For their new experiments, the researchers created a strain of mice in which the UCP2 gene was deleted, or knocked out.

"Sometimes, one of the best ways to find out what a gene does is it eliminate it and see what effect its absence causes," said Sheila Collins, a Duke researcher and member of the international team.

To the researchers' surprise, they found that the knockout mice did not gain weight when fed a high-fat diet, and they reacted normally when challenged by cold temperature. To all outward appearances, the knockout mice seemed to be the same as normal mice. However, further study revealed that the knockout mice demonstrated a heightened activity of macrophages, potent immune system cells responsible for defeating foreign invaders such as parasites and viruses.

In collaboration with a team from Laval University in Quebec City, Canada, the researchers infected normal and knockout mice with Toxoplasma gondii, a parasite known to cause lethal brain infections in mice. While all the normal mice wasted away and died within weeks, all the knockout mice survived, demonstrating that the lack of UCP2 conferred protection against a deadly infection.

Co-senior authors Collins and Daniel Ricquier, Sc.D., of the French Centre National de la Recherche Scientifique (CNRS), published the results of their study in the December issue of the journal Nature Genetics. Both are researchers whose primary research focus was on UCP2 as a possible genetic basis for obesity and diabetes.

"This finding opens a whole new conceptual door - we have found a type of molecule not known to exist a few years ago that now appears to have a major impact on the immune system," Collins said. "The results of this study provide additional insights into how macrophages work, which could have important implications for our immunology colleagues."

Ricquier, who has been investigating uncoupling proteins and obesity for more than 25 years, believes that the series of experiments conducted by the team should lead other researchers to reconsider the role of UCP2 in certain immune diseases in humans.

"We observed that the knockout mice were resistant to infection by parasites because of a more efficient immune system," Ricquier said. "By simply disrupting the UCP2 gene, we were able to make the immune systems of these mice more potent."

While the exact mechanisms behind this are not clear, the researchers found that when compared to normal mice, the macrophages in the knockout mice produced more reactive oxygen species (ROS), highly reactive ions that can wreak havoc within cells.

In the living mouse models, the knockouts produced 80 percent more ROS compared to normal mice, and in an in vitro mouse model, the killing ability of macrophages was five times greater in knockout mice against Toxoplasma gondii. It is well established in immunology, Collins said, that one of the mechanisms immune system cells use to kill invaders are ROS.

For Ricquier, the key question is the relationship between the increased production of ROS and the absence of the UCP2 gene. It appears that a key to understanding this phenomenon lies in the mitochondria, organelles that are located within cells and are considered the "power generator" of all cells, converting food into energy through the process of oxidation.

"It is known that mitochondria can produce ROS, and this production is based on the level of its activation," Ricquier said. "The more energized the mitochondria, the more ROS it produces. We believe that UCP2 somehow plays a role in controlling the energy status of the mitochondria - this is one of our current avenues of investigation."

In more general terms, the researchers believe these findings can provide insights into a host of human diseases and disorders, as well as the process of aging. Reactive oxygen ions are the normal byproducts of cellular metabolism, and if left unchecked, can cause damage to surrounding cells. The body has systems to "mop up" these reactive oxygen ions and neutralize their damaging properties.

"Since cells need a system to control the tendency of mitochondria to over-produce ROS, it could be that the major role of UCP2 is to respond to changing levels of these damaging oxidants and control them," Ricquier said.

The class of UCP genes is an ancient one that has not only been found in mammals, but in plants and fish, according to Ricquier.

"When any cell uses oxygen, it produces a lot of energy, which is a very efficient process for plants and animals," Ricquier explained. "However, the price to pay is the creation of these highly reactive oxidant by-products. So it would make sense to have a system to limit ROS, and these uncoupling genes may play a more important role than we first thought."

"This is another case of finding somewhat unexpected connections between two different areas of research, with potentially important implications for medicine," Collins said.

The research was supported by the National Institutes of Health, CNRS, Association de Recherches sur le Cancer, Institut de Recherches Servier, Association Francaise contr less Myopathies, and the Human Frontier Science Program.

The series of experiments involved toxoplasma gondii were conducted by Drs. Denis Arsenijevic and Denis Richard at the Institut Universitaire de Cardiologie et de Pneumologie, Hopital Laval, in Sainte-Foy, Canada.

For more information, or to contact Duke University Medical Center, see their website at: www.mc.duke.edu