Enzyme might underlie some stroke damage

Stroke treatment is often hampered because the primary available drug, a clot-buster called tPA, is effective only within three hours of a stroke’s onset; many strokes are not detected until after that window has closed.

Knowing the molecular culprits involved in brain tissue damage caused by a stroke might give scientists another angle to exploit as they seek treatments, says Harald Schmidt, a physician and pharmacologist at Maastricht University in the Netherlands. 

To that end, Schmidt teamed with an international group of scientists to study the role of NOX4, a member of an enzyme family that makes free radicals — highly reactive molecules that can kill cells and contribute to tissue damage.

The researchers induced strokes in mice by threading a filament up an artery into the brain to a point at which it obstructed the vessel. In a series of experiments, the scientists found that mice genetically engineered to lack the NOX4 enzyme showed substantially less damage from these strokes than mice making NOX4. In other tests, normal mice with the ability make the NOX enzymes fared better against strokes when given an experimental drug that neutralized NOX4 than did mice not receiving the drug.
NOX4 showed up in brain tissue stressed by stroke in these mice, but the scientists found very little of the enzyme in healthy brain areas.

When the researchers analyzed human brain tissue obtained from stroke patients and stored in European tissue banks, they found the same excess NOX4 in damaged tissues but not in healthy areas, Schmidt says.

Scientists have theorized that the NOX family of enzymes produces free radicals to fight intruders and maintain defenses. NOX2 has been linked to inflammation, which can be protective but can also cause collateral damage. NOX enzymes are suspected to play a role in high blood pressure, aortic aneurysms, heart attack and other conditions, including stroke.

In the new study, the researchers found that knocking out animals’ ability to make NOX1 or NOX2 enzymes didn’t limit stroke damage in mice. Only stopping NOX4 seemed to have an effect, Schmidt says. The NOX4-neutralizing drug, called VAS2870, limits the enzyme’s production of hydrogen peroxide, a harsh free radical.

“This is interesting because no one’s really looked at NOX4,” says Midori Yenari, a neurologist at the University of California, San Francisco School of Medicine. “This whole field is still relatively new.”

She cautions that other approaches to stroke treatment, including those using antioxidants to neutralize free radicals, have also succeeded in animals — only to fail in people.

Schmidt acknowledges the point. “We need to validate these findings in other species,” he says.

Meanwhile, NOX4 itself remains a riddle. “There must be some physiological role for this enzyme. It’s not just sitting around there, waiting to be deleterious in a stroke,” Schmidt says. “But its normal function is still an open question.”