NIH Funds Study of Secondary Injury After Stroke Treatment
After an ischemic stroke, the possibility of serious side effects from reperfusion therapies limits who can receive them. Researchers at Barrow hope to change that.
The National Institutes of Health (NIH) has awarded a $3.6 million R01 grant to Barrow Neurological Institute for a study aimed at understanding how and why some people experience a secondary injury following ischemic stroke treatment—and what we can do to prevent it.
By examining what’s happening in the brain at a molecular level, endovascular neurosurgeon Andrew Ducruet, MD, and neuroscientist Saif Ahmad, PhD, hope to pave the way for a drug that could prevent this secondary injury. Such a drug might allow more people to receive existing therapies aimed at restoring blood flow in the brain, while improving patient outcomes across the board.
Stretched to the Limit
Ischemic strokes, which result from blockages rather than bleeds, account for 87 percent of strokes. Current treatments are designed to restore blood flow in the brain, known as reperfusion, either by dissolving or removing the blockage. Administering a clot-busting medication or using specialized instruments to pull the clot out of the blood vessel can save a person’s life and prevent neurological deficits. However, for some patients, this restoration of blood flow can actually lead to complications, such as brain bleeds and uncontrolled swelling.
This risk for secondary injury restricts who is eligible for reperfusion therapies. For someone whose stroke is too big, too small, or diagnosed too late, the risks of reperfusion may outweigh the potential benefits.
“A brain injured by a stroke can come back to life with reperfusion, but you’re also bringing in a rush of blood and triggering inflammation, so the act of reperfusing the brain can cause damage,” Dr. Ducruet said.
Managing this secondary injury often involves taking patients to the operating room and removing a portion of their skull until the swelling subsides. These patients may experience long-term functional and cognitive difficulties.
When Good Inflammation Turns Bad
This five-year study homes in on the complement system, a group of proteins involved in immunity. These proteins activate in a chain-reaction fashion when the body encounters a foreign invader, such as a virus or bacteria, and work to overpower it.
Studies have shown that the complement system also plays a key role in the development and exacerbation of ischemic stroke. Although the underlying mechanisms remain unclear, the normally helpful infection-fighting inflammation appears to do more harm than good.
“The body’s essentially trying to heal the stroke but, at the same time, it’s over-exuberant,” Dr. Ducruet explained. “It stirs up more swelling and more damage.”
As co-principal investigators, Drs. Ducruet and Ahmad are focusing on a particular aspect of the complement cascade: the C3a receptor (C3aR). Their previous work suggests that this receptor heightens communication between endothelial cells, promoting a breakdown of the blood-brain barrier. This barrier consists of an army of tightly packed endothelial cells that line the brain’s blood vessels. They determine which molecules can enter the brain, ushering in helpful nutrients while denying harmful intruders.
“That’s how we generated this idea: What would happen if we knocked down, or turned off, this receptor from the endothelial cell?” Dr. Ahmad said.
Hitting the ‘Off Switch’
This NIH-funded study does precisely that. Drs. Ducruet and Ahmad have created a unique mouse model, in which they’ve applied gene editing to block the activity of C3aR. They will compare how the brains of these mice and the brains of normal mice respond to tissue plasminogen activator (tPA), a clot-busting drug used to treat ischemic stroke in humans.
The team, which also includes Arizona State University clinical assistant professor Kanchan Bhatia, noted three specific aims for their study. The first is to demonstrate that inhibiting C3aR protects the blood-brain barrier. The researchers can assess this hypothesis by examining whether immune cells, such as myeloid cells, have crossed the blood-brain barrier in the genetically edited mice.
Secondly, they intend to show that C3aR deletion protects the brain from secondary injury after restoration of blood flow. This will involve evaluating the brains of mice for bleeding, swelling, and neurological dysfunction.
Lastly, the researchers will probe nerve cell activity to test their hypothesis that C3aR is responsible for gobbling up connections between nerve cells, called synapses, in the post-ischemic stroke brain. They also hope to show that blocking the receptor has the potential to improve long-term functional outcomes.
“A big part of this work is looking at long-term cognitive and behavioral outcomes because there’s another, less-defined role for complement, and that’s in clearing synapses,” Dr. Ducruet explained. “We think an aberrant function in the synaptic clearance can lead to functional problems down the road in terms of cognitive outcomes.”
If successful, this study would be the first to define a link between C3aR-associated inflammation and myeloid-mediated synaptic dysfunction after stroke.
Drs. Ducruet and Ahmad anticipate that turning off C3aR will both abate secondary injury after reperfusion treatment and improve long-term function associated with post-ischemic synapse elimination in aged mice.
A Foundation for Drug Translation
By illustrating the role of C3aR in secondary injury after reperfusion, this study could set the stage for trials of a drug blocking the receptor’s activity.
“Our hope is maybe it would allow us to give reperfusion therapies to more patients and protect against the side effects that are otherwise prohibitive,” Dr. Ducruet said.
He and Dr. Ahmad have already identified a promising small-molecule inhibitor and are applying for separate funding to investigate it further.
“We’re trying to march that down the translational line—from cells, to animals, and ultimately humans,” Dr. Ducruet said. “So this grant represents the nuts-and-bolts basis for translation of a strategy like this.”