Finding Could Lead To New Drug Targets Against Stroke, Other Diseases
BOSTON--September 25, 1998--Researchers at Harvard Medical School have discovered an early step in the progression of stroke, a finding that could provide a new target for antistroke therapies. Stroke afflicts an estimated half million Americans each year. The study is published in the September 29 Proceedings of the National Academy of Sciences.
Scientists believe that while some brain cells are killed almost immediately in stroke by the blocking or bursting of blood vessels, others may die through exposure to compounds released by the dying neurons. Among these is glutamate, a substance that excites target neurons. One way to limit the second wave of destruction would be to stop glutamate from killing the cell. Yet the identity of the specific intracellular routes by which glutamate works its deadly, or 'excitotoxic,' effects has eluded researchers.
Beverly Murray, a research fellow in neurology at Harvard Medical School and the paper's lead author, Ed Furshpan, a professor of neurobiology at Harvard Medical School, and their colleagues have discovered that a member of the MAP kinase family known as ERK, famous for its role in such life-promoting activities as cell growth and division, may act as a kind of molecular gatekeeper, activating one or more excitotoxic pathways. Few would have suspected such a connection. "People thought the influx of glutamate activates calcium which, in turn, activates kinases. But the pathway was very fuzzy," says Furshpan.
The unexpected finding was made by mixing cultured brain cells with an ERK-blocking drug and then inducing seizures in the cells. Like stroke, seizures are believed to cause cell death through the release of excess glutamate. The researchers found the ERK-blocking drug, called PD 098059, protected cells from the deadly effects of glutamate.
"The exciting thing about this is not only that it deciphers an initial stage in the pathway of stroke, but also suggests that a pathway thought to be involved in cell proliferation may lead to damage early on in ischemia," says Alessandro Alessandrini, assistant biologist at Massachusetts General Hospital and co-author on the PNAS paper.
Intriguingly, Alessandrini and his colleagues have injected the ERK-blocking drug into seven mice, prior to blocking their cerebral blood vessels. On average, the region of cell death caused by the ensuing stroke was reduced 55 percent compared to mice who did not receive the drug.
In the experiments, PD 098059 was administered before or at the very beginning of the stroke or seizure. "So it might not be a good inhibitor once stroke is in progress," says Alessandrini. But the findings do suggest other ways to stop stroke once it has begun. For example, drugs designed to bind activated forms of ERK might prevent them from delivering their death-dealing signals. "You could also imagine designing a drug that could dephosphorylate, or deactivate, the MAP kinase," says Murray.
The recent findings join two prime scientific topics--stroke and the MAP kinases--that previously displayed only a hint of connection. Gerbils suffering from global ischemia--the blockage of blood that often leads to stroke--had been found to display high levels of MAP kinase. Intrigued by this finding, Allessandrini began his experiments in mice.
Murray and Furshpan had been using cultured cells from the hippocampus, a region of the brain prone to stroke and seizure, to study glutamate's toxic effects. They were looking for a way to block those effects when Alessandrini approached them with the idea for a collaboration.
Normally, when neurons undergo seizure for 30 minutes--with the accompanying release of glutamate--30 to 50 percent of hippocampal cells died or exhibited damage within 24 hours. Cells mixed with PD 098059 before and during seizure exhibited almost no damage. "We were surprised. It was a very dramatic effect." says Furshpan.
"What is especially exciting is that this basic excitotoxic mechanism may be important for many types of neurodegenerative disease," says Murray.
Journal
Proceedings of the National Academy of Sciences