By David N. Leff

A bony helmet called the skull shields your brain from blows to the head, and other cranial accidents. But when a stroke leads to topping up the cranial cavity with fluid, that same skull acts as a cofferdam to cruelly trap the edema-squeezed brain inside.

More than half a million Americans a year suffer a stroke - and 160,000 deaths result. One-fourth of those who have a stroke die within a month. By the year 2050, the number of stroke victims is expected to double. Risk factors for stroke closely parallel the same lifestyle habits that bring on atherosclerosis and coronary heart disease. They come in two types, treatable and untreatable. High on the treatable list are excessive use of alcohol, tobacco and illicit drugs, hypertension and diabetes. Untreatable stroke risk factors include age over 60, male gender, and a family history of stroke.

There's only one medical treatment for stroke - the clot-busting tissue plasminogen activator or streptokinase - the same drugs of choice that treat heart attacks. But this costly antithrombolytic therapy carries its own severe side effects and uncertain outcome.

Today, scientists at the Scripps Research Institute in La Jolla, Calif., unveiled a novel strategy for preventing or mitigating stroke. Their report, in the February issue of Nature Medicine, is titled: "Src deficiency or blockade of Src activity in mice provides cerebral protection following stroke." The article's lead author is cell biologist David Cheresh, professor of vascular biology and immunology on the Scripps faculty.

"Our overall finding," Cheresh told BioWorld Today, "is that we were able to discover, and interfere with, a pathway involved in vascular permeability. VP leads to very serious consequences for stroke patients, first, the increase in edema, which is well known to occur following stroke. And that edema," he added, "causes additional massive brain damage, which is essentially irreversible."

When a blood clot lodges in a cerebral artery that feeds oxygen and nutrients to the brain, the resulting blockage turns on vascular endothelial growth factor (VEGF), a molecule that springs into action wherever tissues are threatened with hypoxia - oxygen starvation. VEGF's day job is to propagate endothelial cells, which line blood vessels with seamless inner walls.

Oxygen Starvation Starts Ball Rolling

But faced with hypoxia, VEGF opens that dam. Its molecular alias, vascular permeability factor (VPF), allows blood and water to leak out of the arteries, flout the blood-brain barrier, and flood the brain's cerebral space. Cheresh explained: "VPF is one of stroke's complications. As it does for any hypoxic tissue damage - ischemic brain injury, myocardial infarction, anywhere there's a loss of oxygen in tissue - VEGF/VPF will be overproduced.

"The endothelial cells pull apart from one another somewhat," Cheresh observed, "and open up transiently. VEGF/VPF breaks down that tight-junction blood-brain barrier, allowing proteins from blood to leak out, and also water to accumulate outside the vasculature. In stroke it has the adverse consequence that leads to hyperpermeability of those blood vessels."

What happens in the brain, which is surrounded by a very hard skull, is that the increase in edema - of water flowing into the brain - causes swelling. And that swelling against the hard skull simply increases the intracranial pressure.

"This adds to the injury by cutting off the blood flow to other parts of the brain. So it increases the actual level of ischemia - loss of blood flow. You could imagine," Cheresh continued, "that if you were to simply increase the pressure of the brain to the point where the blood couldn't move very well, the amount of damage, and the amount of brain that was exposed to low or negligible oxygen, would increase. And that loss of oxygenated blood flow causes the devastating brain damage associated with stroke that leads in many cases to death or massive debilitation."

A key perpetrator of ischemic stroke is a molecule called Src (pronounced sark).

"Src is just a three-letter word," Cheresh observed, "that represents a family of enzymes involved in signal transduction. Kinases transfer phosphates from, say, ATP [adenosine triphosphate] to a protein, which allows information to flow from outside of the cell to its interior. So when VEGF binds to the endothelium," he went on, "it turns on Src activity. This in turn, as we report, activates events in the endothelium that cause that permeability breakdown, which leads to the edema that follows the stroke.

"Src's normal function in the body," Cheresh pointed out, "is to regulate cell proliferation and migration, wound repair and, of course, vascular permeability and angiogenesis."

The co-authors conducted two in vivo experiments to see how curbing Src might reverse stroke.

Mice Trump Src Two Ways

"In one experiment," Cheresh recounted, "we used knockout mice deficient in the Src gene. When we induced a stroke in those mice, by occluding their middle cerebral artery, we noticed that the level of long-term damage in their brains was much less. They had minimal permeability, no edema, no loss of vessel integrity - all of which the control Src-plus mice did experience.

"In a second approach," he went on, "we tested normal wild-type animals that had Src activity, just like a human being does. And after creating the stroke, we followed up six hours later by injecting them in the bloodstream with a Src-inhibiting compound. We found that the inhibitor did effectively the same thing that we saw in the Src-minus animals: It protected them from stroke.

"The anti-Src inhibitor we used," Cheresh explained, "is a small organic molecule called PP1, that is under development by Pfizer as an anticancer drug.

"Our goal," he stated, "is to develop Src inhibitors - either something like PP1 or something different - for human patient studies of ischemic stroke. Scripps has filed a use patent, of which I am an inventor," Cheresh went on, "covering any inhibitor of Src kinase, to suppress VP and related tissue damage.

"The outlook is," he concluded, "that we would like to get a corporate partner, which would help us fund the necessary safety trials, and some additional preclinical efficacy experiments, to make our way toward Phase I clinical trials in the next 18 months."