In experiments with genetically engineered mice, Johns Hopkins researchers have found an "off-switch" for systemic inflammation, the body's overall response to injury and infection. The findings may have implications for treatment of inflammation-related diseases in humans, from autoimmune disorders to atherosclerosis, the researchers say.
Unlike the case in normal mice, in mice bred not to have the "off-switch," an injection of inflammation-causing proteins led to fatal kidney failure, the result of uncontrolled systemic inflammation. Studies have also revealed which genes were affected by the missing off-switch, a protein called stat3-beta, says Stephen Desiderio, an author of the report in the Feb. 8 issue of Cell.
Future studies may prove some of those affected genes, or even stat3-beta itself, to be good targets for treating abnormal systemic inflammatory responses, the researchers say. In humans, systemic inflammation is reflected in general malaise, the sensation of not feeling well.
"This general inflammation helps keep us alive until the immune system builds specific tools like antibodies to fight off germs or other invaders," says Desiderio, a Howard Hughes Medical Institute investigator at Hopkins and professor of molecular biology and genetics.
"It takes a whole host of molecules to respond to the initial injury or infection, but that response can be lethal if it goes on too long. In the mice we studied, stat3-beta shuts off that response," he adds. "Stat3-beta's role in humans is likely to be quite similar, although the details will no doubt differ."
In both mice and humans, stat3-beta regulates various genes, turning them on or off. The researchers found that in normal mice a sharp spike in stat3-beta's activity comes shortly after inflammation begins. In mice engineered to lack stat3-beta, experiments showed that more than 120 genes were expressed at higher levels than normal, suggesting the protein normally turns off these genes.
To carry out these studies, postdoctoral fellow Joo-Yeon Yoo carefully altered the stat3 gene to create a mouse missing only stat3-beta, a tricky project because the gene also codes for another, longer protein called stat3-alpha.
When mice were injected with bacterial proteins, an established technique to cause systemic inflammation, those missing stat3-beta did not recover. Instead, those mice fairly rapidly developed kidney failure as a result of raging inflammation, explains Desiderio.
"In normal mice, the immune system initially kicks up its activity, creating systemic inflammation, and then backs off," he says. "Mice without stat3-beta, however, couldn't turn off the initial response to the injection, and that inflammatory reaction eventually caused irreversible damage to the animals' own tissues."
The scientists then turned to microarray technology to determine the genes affected by stat3-beta's absence. Using a microscopic grid of 12,000 markers--a microarray--the scientists found 128 genes expressed differently, most at higher levels, in mice without stat3-beta. Microarray technology lets scientists examine the activity of a large part of an organism's genome; this study tested about a third of the mouse genome.
Desiderio says mice lacking stat3-beta can be used to explore the relationship between chronic inflammation and various conditions; even cardiovascular diseases like atherosclerosis seem to have an inflammatory component, he says.
Stat3-beta also helps explain humans' complexity, say the scientists. The protein results from a process called "alternative splicing" that acts as an editing machine for the raw film of the genome. Just as film reels can be edited multiple ways to get different movies, the information from a single gene--actually the resulting RNA--can be edited differently to create proteins with different structures and functions.
"If we don't have many more genes, why are we so much more complex than worms?" Desiderio asks. "One answer is alternative splicing--if each protein doesn't have to come from a distinct gene, our collection of proteins can be more complex than the number of genes would seem to allow. Even more so, alternative splicing of a regulatory gene like stat3 can lead to changes in expression of a large number of target genes, amplifying its effect." Initial estimates for human genes were in the hundred thousands to match estimates of our proteins, but the draft of the human genome released a year ago cut the number of genes to only 30,000 or so.
Other authors on the report are David Huso of the Division of Comparative Medicine at Johns Hopkins and the late Daniel Nathans of Molecular Biology and Genetics and the HHMI. All funding was provided by HHMI.