By KRISTA CONGER
Protecting the nation against bioterrorism may one day be just a bit easier, thanks to a new $7 million government grant recently awarded to medical center researchers.
The money will be used to study a small molecule called granulysin, which is normally used by the immune system to attack microbial invaders or tumor cells. The researchers hope to harness the molecule’s killing activity to fight organisms that might be used in bioterrorism or bacteria that are resistant to conventional antibiotics.
"Obviously the government is very concerned that terrorists might engineer microbes — even those that are not highly lethal themselves — to deliver lethal agents," said Carol Clayberger, PhD, professor of pediatrics and the grant’s principal investigator. "And antibiotic resistance is a growing public health problem."
Granulysin was discovered about 15 years ago by a team of Stanford researchers, including Alan Krensky, MD, professor of pediatrics and chief of pediatric immunology and transplantation biology at Lucile Packard Children’s Hospital. The scientists had been looking for genes that were expressed by T cells of the immune system that were in full attack mode, several days after activation by antigens.
"One of the reasons that we were so interested in granulysin," said Clayberger, "is that it can kill the organisms that cause tuberculosis and malaria in addition to a wide variety of other microbes."
The researchers found that granulysin, which is stored in granules inside the immune cells, acts by latching on to the cell membrane. It burrows deep to release the contents of the cell, or, in mammalian cells, induces a common "self-destruct" pathway called apoptosis. This novel way of killing may be granulysin’s biggest selling point.
"One of the big problems today is that many organisms are becoming resistant to antibiotics," said Clayberger. "Granulysin probably won’t be affected by this problem. Most antibiotics target a specific bacterial enzyme or molecule, which can mutate to escape the drug. Because granulysin works in a much broader way — targeting the cell membrane — it would be very difficult for a microbe to become resistant to granulysin."
With an eye toward developing a version of granulysin that could be used in humans, Clayberger and her colleagues began tinkering with small pieces, or peptides, of the molecule.
"We’ve managed to engineer short peptides that are still active," said Clayberger. "And by making small amino acid changes we have made them specific for microbes and kept them from killing mammalian cells." Without such a screening mechanism to discern friend from foe, granulysin’s usefulness as a possible human therapy might be derailed.
"Now we’re trying to make the peptides even more potent," said Clayberger. "We’re planning to experiment with different amino acid insertions, testing them for activity in the lab. Once we identify the best candidates, we will explore ways to further modify them to make them even more effective. We’ll also use several different animal models to identify exactly how granulysin and its derivatives cause cell death through the apoptotic pathway."
In addition to Clayberger and Krensky, collaborators on the five-year grant from the National Institute of Allergy and Infectious Diseases include Gary Schoolnik, MD, professor of medicine and of microbiology and immunology, and Tom Wandless, PhD, assistant professor of molecular pharmacology.
Stanford Report, November 12, 2003