Scientists from the University of Utah have discovered that preventing the formation of a sticky, ‘web-like’ substance that can form in blood vessels after a stroke could protect the brain and aid in stroke recovery.
What is a stroke?
Stroke is a leading cause of disability in the US. The most common form – ischemic stroke – occurs when a vessel-blocking clot impedes the flow of blood to the brain. Quick treatment to break up or remove the clot can restore blood flow, limit damage to the brain, and contribute to stroke recovery.
However, according to this recent study, the blood that comes rushing back carries cells with the potential to cause further harm. Brain damage can worsen even after a clot has been eliminated when immune cells in the blood release sticky webs, known as neutrophil extracellular traps (NETs), that further clog vessels.
“Those NETs can gum up the vessels [in the brain] by trapping other cells and reducing the amount of blood flow, causing more brain injury,” explained Robert Campbell, Senior Author, PhD, and research Assistant Professor of Internal Medicine. “Markers of NETs correlated with poorer stroke outcomes in patients that we saw here at the University of Utah.”
Campbell and his colleagues discovered that they could prevent these effects in a mouse model of stroke by treating animals with a NET-blocking compound.
“We desperately need new treatments for ischemic stroke,” added Jennifer Majersik, co-author, MD, a Professor of Neurology, and Stroke Specialist at Utah Health. “At the most, only 30% of patients are eligible for the standard therapies currently available to open up a blocked blood vessel.” She added that taking a new look at what causes and can prevent brain injury in patients with stroke is an important step toward better therapies for stroke recovery.
This research was reported in the Journal of Clinical Investigation and led by scientists at the University of Utah Health.
NETs: How can blocking them aid in stroke recovery?
NETs are part of the immune system’s defence against pathogens. White blood cells called neutrophils can release webs, which are constructed of long strings of DNA and bits of a toxic protein, to ensnare viruses and bacteria. However, these can also cause damage to the body’s own tissues.
Scientists have gathered evidence that demonstrates how NETs help drive the development of dangerous blood clots in a range of conditions like sepsis – a life-threatening inflammatory condition – and COVID-19. The research team considered whether they might be involved in stroke, as well.
The team first investigated signs of NETs in the brains of patients who had suffered from a stroke. They examined post-mortem tissue samples that had been preserved in the National Institutes of Health NeuroBioBank and discovered neutrophils, a type of immune cell, in regions impacted by stroke. Some of the cells appeared to have been actively producing NETs at the time of the patient’s death, and inside blood vessels, the NETs served as traps, where cells passing through stalled and built up into a new blockage.
“They are able to trap a lot of other cells and potentially reduce the amount of blood flow that is going into the tissue and how much oxygenation is going into the brain,” Campbell said. “They are very small and fragile, but they can do a fair amount of damage.”
What negative impacts can NETs have?
When the research team analysed blood samples from patients treated for ischemic stroke at the University of Utah Health, they discovered more evidence that NETs might have negative consequences for patients in stroke recovery.
Proteins in the blood indicated that NET production was high compared to the blood of healthy individuals. Additionally, the functional impairments that were caused by patients’ strokes were most severe in individuals whose blood tests at the time of hospital admission indicated the most NET production.
These observations prompted Campbell and Postdoctoral Researcher Frederik Denorme, PhD, lead author, to return to the lab to determine how NETs formed after stroke. To simulate stroke in mice, they temporarily blocked a brain-supplying blood vessel, then allowed blood flow to resume.
In the 24 hours after the procedure, elevated levels of NETs were detectable in both the brain and blood. Neutrophils’ enhanced production of NETs seemed to be triggered by another type of blood cell, the platelets.
“We really think that the time when blood flow is initially restored to the brain is when you get platelet activation and that those platelets then go and activate neutrophils,” noted Campbell.
This resulted in vessel-clogging NETs, which obstructed blood flow, and initiated brain damage beyond that caused by the original clot.
How did scientists avoid this issue?
While NETs role in stroke is considerably concerning, it also suggests an opportunity to intervene. Campbell and his team experimented with giving mice nNIF, a NET-blocking peptide discovered by University of Utah Health Neonatologist, Christian Con Yost, MD.
They discovered that if they prevented NET formation by administering nNIF soon after a simulated stroke, they could protect the brain, and aid in stroke recovery. Mice that received this treatment experienced less brain damage due to stroke than animals that did not receive nNIF, exhibiting better neurological and motor function weeks after the event.
Scientists noted that the treatment was encouraging, improving outcomes not just in young, healthy mice but also in old mice and mice with diabetes. This is important because diabetes and other conditions that become more common as we age can make the vasculature in the brain less healthy and more resistant to treatments.
Cambell observed that the nNIF peptide, which is currently being explored as a potential treatment for sepsis, might help limit stroke damage for patients, too. The peptide cannot stop NETs once they have formed, but the findings in mice suggest there may be a window of opportunity during which it is possible to intervene and prevent the formation of the toxic webs.
Scientists have noted that additional research is required to further evaluate nNIF’s effects in animal models of stroke before determining whether it is appropriate for clinical testing in the context of stroke recovery treatment.