The surveillance system of a cell rivals that of any bank vault: a complicated configuration of sensors, tripwires, and alarms. Molecular guards patrol the periphery and interior of cells to lock out, inactivate, or destroy harmful particles. It takes a cunning invader to cross the physical barriers and elude second- and third-line sensors.
Cell biologists have long understood some components of the security system while others remained hidden in a tangle of cellular wiring. Now, HHMI investigator Zhijian “James” Chen of the University of Texas Southwestern Medical Center has uncovered a mechanism the cell uses to detect and react to signs of invading organisms—specifically, loose pieces of DNA inside the cell. The discovery helps explain a basic cellular function and sheds light on how the biological surveillance system is poorly wired in some autoimmune diseases.
Even before scientists knew the role of DNA and RNA in transmitting genetic information, they knew that these molecules could trigger immune responses in humans. More recently, scientists learned that if DNA or RNA of microbial pathogens, such as viruses and parasites, entered the watery interior cytosol of a cell, the immune system would be called into action to fight the invaders. In some cases, however, a cell’s own DNA is found loose in the cytosol rather than contained in a central nucleus or organelle, and this loose DNA can trigger autoimmune reactions that cause diseases such as lupus.
“The cytosol has to be very clean,” says Chen. “If DNA gets in, that is actually a danger signal to cells.”
After discovering how molecules patrol the cytosol for loose RNA, Chen and his colleagues wanted to know how cells detect rogue DNA. So they added bits of DNA to cytosol that had been removed from cells and then fractionated, or divided, the liquid mixture into different components. By inserting each component into fresh cells and running gene expression assays, the scientists could determine which fraction turned on immune system activation genes—in other words, which one contained the molecules responsible for sensing the loose DNA.
“Using this method, we could repeatedly fractionate the samples to narrow down our search for the active components of the system,” says Chen. “This assay was really the key to our discovery.”
Through repeated narrowing down of the fractions, followed by analytical chemistry, Chen’s lab group revealed that two molecules are critical to detecting loose DNA. Further biochemical experiments helped them understand the roles of the molecules in sensing DNA and setting off an immune response.
An enzyme called cyclic GMP-AMP synthase (cGAS), they discovered, binds directly to any DNA that’s free-floating in the cytosol—it’s the initial sensor of free DNA. “The cGAS enzyme recognizes all double-stranded DNA without any apparent sequence specificity,” says Chen. “Which makes sense because an organism wants to recognize all sorts of DNA from different sources.”
Once cGAS is bound to DNA, it becomes active as an enzyme, a protein that can carry out chemical reactions. In this case, when cGAS is activated it synthesizes a chemical messenger called cyclic GMP-AMP (cGAMP). The newly made cGAMP binds to a protein called STING, which turns on a cascade of immune reactions.
The scientists’ findings, reported in two papers published online in Science on December 20, 2012, could also launch a new generation of treatments for some autoimmune disorders. In diseases such as lupus and Sjögren’s syndrome, the immune system is in a state of constant activation—as if a bank vault’s alarms ring incessantly even when all is clear. One trigger of the constant immune reaction in these diseases may be the loose DNA in cells. So it’s likely that cGAMP and cGAS are abnormally active, Chen says, in people with these diseases. More research is needed to fully understand the roles the molecules may play in disease, but it’s possible that blocking their activity could block the constant immune response, Chen says.
“We hope we can go on to identify chemical inhibitors of this enzyme,” says Chen. “Such inhibitors could be developed into therapies for autoimmune diseases.”
For now, Chen wants to work out more of the molecular details of the loose DNA alert system and test its role in animal model systems. He hopes to learn ways to hack the molecular alarms, to coax the cell to let down its guard in some cases, and to help cells strengthen their defenses against intruders in others.