A new study has prompted scientists to reconsider a once popular but controversial idea in stroke research.
Neuroscientists thought that calming over-excited neurons after a stroke might stop them from releasing a toxic molecule that can kill neurons already damaged by lack of oxygen. This idea was supported by studies in cells and animals, but lost favor in the early 2000s after numerous clinical trials failed to improve outcomes for stroke patients.
But a new approach has provided evidence that the idea may have been too hastily dismissed. The new findings are available online in the journal Brain.
By scanning the entire genomes of nearly 6,000 people who had suffered a stroke, researchers at Washington University School of Medicine in St. Louis identified two genes linked to recovery within the crucial first 24 hours after a stroke. Events, good or bad, that occur on day one put stroke victims on the road to long-term recovery. Both genes turned out to be involved in regulating neuronal excitability, providing evidence that overstimulated neurons influence stroke outcomes.
“There has been this ongoing question of whether excitotoxicity is really important for stroke healing in humans,” said co-senior author Jin-Moo Lee, MD, PhD, Andrew B. and Gretchen P. Jones Professor and Chief of the Department of Neurology . “We can cure a stroke in a mouse with excitotoxicity blockers. But we’ve done a lot of clinical trials in humans, and we couldn’t move the needle. Every single one of them was negative. In this study of 20,000 genes, the top two genetic hits indicate mechanisms involving neuronal excitation. That’s pretty remarkable. This is the first genetic evidence showing that excitotoxicity is important in humans and not just in mice.”
Each year, nearly 800,000 people in the United States suffer an ischemic stroke, the most common type of stroke. Ischemic strokes occur when a clot blocks a blood vessel and cuts off oxygen to a part of the brain, causing sudden numbness, weakness, confusion, difficulty speaking, or other symptoms. Over the next 24 hours, symptoms continue to worsen in some people, while symptoms stabilize or improve in others.
In the 1990s, Dennis Choi, MD, PhD, then chief of the Department of Neurology at Washington University, conducted pioneering research on excitotoxicity in stroke. He and others showed that a stroke can cause neurons to release large amounts of glutamate, a molecule that transmits excitatory messages between neurons. Glutamate is constantly being released from neurons as part of the normal functioning of the nervous system, but too much at once can be toxic. Efforts to translate this basic research into human therapies failed, and eventually pharmaceutical companies phased out their anti-excitotoxic drug development programs.
But Lee, who had previously worked with Choi on excitotoxicity, didn’t give up. He teamed up with genetics researcher and co-senior author Carlos Cruchaga, PhD, the Barbara Burton and Reuben M. Morriss III professor of neurology, and a professor of psychiatry; First author Laura Ibañez, PhD, Assistant Professor of Psychiatry; and co-author Laura Heitsch, MD, assistant professor of emergency medicine and neurology, to address what drives brain injury after stroke. The team identified people who had suffered strokes and looked for genetic differences between those who naturally regained essential function on day one and those who didn’t.
As members of the International Stroke Genetics Consortium, the research team was able to study 5,876 ischemic stroke patients from seven countries: Spain, Finland, Poland, the United States, Costa Rica, Mexico and South Korea. They measured each person’s recovery or worsening on day one by the difference between their scores on the National Institutes of Health (NIH) stroke scale six and 24 hours after the onset of symptoms. The scale measures a person’s level of neurological impairment using measures such as their ability to answer basic questions like “How old are you?”; to perform movements such as holding the arm or leg up; and feeling when touched.
The researchers performed a genome-wide association study by searching the participants’ DNA for genetic variations linked to the change in their NIH stroke scores. The top two hits were genes that encoded the ADAM23 and GluR1 proteins. Both relate to the sending of excitatory messages between neurons. ADAM23 forms bridges between two neurons, allowing signaling molecules such as glutamate to be passed from one to the other. GluR1 is a receptor for glutamate.
“We started with no hypotheses about the mechanism of neuronal damage,” Cruchaga said. “We started out by assuming that some genetic variants are linked to stroke recovery, but we didn’t guess what they were. We tested every single gene and every genetic region. So the fact that an unbiased analysis revealed two genes involved in excitotoxicity tells us it must be important.”
In the years since antiexcitotoxic drug development was abandoned, anticoagulant drugs have become the standard of care for ischemic stroke. Such drugs aim to restore blood flow so oxygen — and everything else in the bloodstream, including drugs — can reach the affected brain tissue. Consequently, experimental neuroprotective therapies that have failed in the past may now be more effective because they have a better chance of reaching the affected area.
“We know that this first 24-hour period has the greatest impact on the results,” Lee said. “Beyond 24 hours, there are diminishing results in terms of impact on long-term recovery. At the moment we don’t have any neuroprotective agents for those first 24 hours. Many of the original studies of anti-excitotoxic agents were conducted concurrently when we were unsure of the best study design. We’ve learned a lot about stroke over the past few decades. I think it’s time for another review.”