SAN FRANCISCO, CA — Days and even years after someone has suffered a stroke or traumatic brain injury, they are at an increased risk of developing epilepsy. Now researchers at Gladstone Institutes found that star-shaped cells called astrocytes in the thalamus play a key role in predisposing seizures in brain-injured mice.
The team also analyzed human post-mortem brain tissue and showed that the same cells identified in mice may have been altered in the thalamus of people affected by brain injury and stroke. The findings, published in the journal Science Translational Medicine, suggest that targeting a protein in these cells could prevent the long-term damage from brain injury.
“In the wake of brain injury, the thalamus has been relatively understudied compared to other brain regions,” says Jeanne Paz, PhD, an associate researcher at Gladstone and senior author of the new study. “I hope this is just the beginning of many new lines of research about how important this region is in determining how we can help the brain be resilient to the effects of injury.”
A cascade of inflammation deep in the brain
At the time of a stroke or traumatic brain injury, many cells at the site of the injury die almost immediately. Inflammatory cells and molecules begin to collect and clean up the dead cells and molecular debris. In the thalamus, an area deep in the center of the brain that may be far from the site of injury, cells called astrocytes are activated, leading to a cascade of inflammatory changes.
Previous studies by the team have shown in rodent models that activation of astrocytes in the thalamus is a common consequence of brain injury. However, astrocytes also play a key role in supporting neurons, including controlling their connections and providing them with nutrients.
In this study, the scientists wanted to determine whether activation of astrocytes in the thalamus helps the brain repair, leads to additional damage, or has both positive and negative effects.
“Astrocytes are so important to the brain that you can’t just remove them to treat disease,” said Frances Cho, a graduate student at Gladstone and UC San Francisco (UCSF), and the lead author of the new study. “We had to determine whether we could separate the harmful actions of activated astrocytes from their protective actions.”
Focus on the Thalamus
Paz, Cho and their collaborators hypothesized that activated thalamic astrocytes might play a role in some of the long-term symptoms of brain injury, including an increased risk of seizures and sleep problems. So instead of studying mice with brain injuries, the team initially tested the effects of activating thalamic astrocytes in otherwise healthy animals. They found that activating thalamic astrocytes alone was enough to induce altered brain activity patterns similar to those following injury, and predispose the mice to seizures.
Then, when the researchers analyzed the molecular properties of the activated astrocytes, in collaboration with the team of Anna Molofsky, MD, PhD, at UCSF, they found that these cells lost a protein called GAT3, which is responsible for regulating the levels of a specific inhibitory neurotransmitter molecule. As a result, the neighboring neurons were exposed to too much of the neurotransmitter, resulting in neuronal hyperexcitability and susceptibility to seizures.
“We wondered if the loss of GAT3 in thalamic astrocytes caused neuronal dysfunction, increasing the level of this protein could solve the problem and restore the function of the neurons?” says Paz, who is also an associate professor at UCSF.
To answer this question, the team collaborated with Baljit S. Khakh, PhD, and his group at UCLA, who had developed a tool to specifically increase GAT3 in astrocytes. Remarkably, increasing GAT3 levels, specifically in thalamic astrocytes, was sufficient to prevent neuronal hyperexcitability and an increased risk of seizures caused by activated astrocytes.
The team then tested whether the results were true in mice with brain injuries. Increasing levels of GAT3 in the thalamic astrocytes of these mice also reduced the risk of seizures and the death rate.
“These activated astrocytes are very different in many ways from astrocytes that are not activated, so it was surprising that we could pinpoint a single molecular change that we could target to prevent the consequences of brain injury,” says Cho.
A potential therapeutic agent
Human thalamus samples are rarely collected during post-mortem brain biopsies. But in collaboration with Eleonora Aronica, MD, PhD, and her group at the University of Amsterdam, the researchers were able to obtain a small number of postmortem thalamus samples — three from people with no known brain injury, three from people who had suffered a stroke, and four from people. with traumatic brain injury.
“The postmortem brains with stroke and traumatic brain injury appeared to have lower levels of GAT3 in their thalamic astrocytes, just as we had seen in the mouse model,” says Cho. “We hope that with more focus on the thalamus, it will become more routine in the future to collect thalamus samples from post-mortem biopsies.”
The researchers hope to continue collecting long-term data on both mice and humans to study the time course of astrocyte activation in the thalamus after brain injury.
“Since these changes in the thalamus occur after the initial brain injury, there is a period of time in which clinicians can intervene to stop or reverse them — and prevent the increased risk of developing epilepsy,” says Paz.
About the research project
The article “Enhancing GAT-3 in thalamic astrocytes promotes resilience to brain injury” was published in the journal Science Translational Medicine on July 6, 2022.
Other authors include Yulia Voskobiynyk, Allison Morningstar, Bryan Higashikubo, and Agnieszka Ciesielska of Gladstone; Ilia Vainchtein and Francis Apparicio of UCSF; Diede Broekaart, Jasper Anink, Erwin van Vliet and Eleonora Aronica from the University of Amsterdam; and Xinzhu Yu of UCLA.
The work was supported by funding from the National Institute of Neurological Disorders and Stroke (F31 NS111819, R01 NS096369, R01 R01NS121287, R00 NS078118, R35 NS111583), the National Institute of Mental Health (DP1 MH104069, DP2 MH116507, R01 MH119349), the National Cancer Institute (P30 CA082103), the National Science Foundation (1144247), a UCSF Discovery Fellowship, Department of Defense (EP150038, EP190020), a Gladstone Institutes Animal Facility grant (RR18928), Pew Charitable Trusts, a seventh EU Framework Program EPITARGET grant (602102), an EU Horizon 2020 research and innovation program Marie Sklodowska-Curie grant (722053) and the Netherlands Epilepsy Foundation (project 16-05).
About Gladstone Institutes
To make sure our work does the greatest good, Gladstone Institutes focuses on conditions with profound medical, economic and social consequences: unresolved diseases. Gladstone is an independent nonprofit life science research organization that uses visionary science and technology to overcome disease. It has an academic affiliation with the University of California, San Francisco.
Science Translational Medicine
Enhancement of GAT-3 in thalamic astrocytes promotes resilience to brain injury in rodents
Article publication date
July 6, 2022